Addic7ed: Subscript Guideline

Never mind 40 to 42 characters per line rule. Use as many as it takes. Reason - The same reason for rule #4 (see below).
Never use a word if it is not actually pronounced. Don't miss a word if it is spoken. Reason - Closed Caption is often mistaken. Simply because it is not based on actual script or CC maker is wrong. Subscript is to help understand what actors actually said. This is the most important rule of all. Most of so called "corrected" subscripts are, in fact, not compliant to this rule. They are not "corrected" actually.
Type as accurately as possible in a way it is pronounced. Reason - Sometimes the words are spoken to be shortened, such as "would've" and "couldn't've" instead of "would have" and "could not have." This rule is to save space as well as to let viewers to understand how actually the words are spoken.
Make each part only 1 line no matter how long it is except dialog. Reason - Breaking a line in middle and making it more than 1 line uses at least 2 lines to show the script forcefully, which wastes the screen vertically. Letting a player handle a long line and automatically break the line is more efficient way to display the long line. A recommended player is MPC-HC (Media Player Classic Home Cinema with ffdshow). Setting - In ffdshow video decoder configuration, go [Subtitles]-[Text]-[Text Subtitles] and check on [Split long subtitle lines] and choose [Smart wrapping, lines are equally broken].
When a person says a certain word, the word must be present in a line shown at the time of the speech. Also, don't start a line before the line is actually started. Don't let a line stay longer than the line has been already said. Reason - If this rule is broken, the subscript is considered as asynchronized one, meaning out of sync. If a line is way ahead of the voice, say 2-3 seconds, it spoils the scene, because viewers are able to see what is going to happen. If a line stays longer, it is simply annoying. Who needs just a word "Hi." stays 5-8 seconds long? Each line has to be there when and only when the line is actually being spoken. Acceptable gap is up to 0.2 second earlier and later, ideally less than 0.1 second.
When a sentence is split into more than 2 lines, use "..." at the end of a line. Don't use "," instead. Reason - "," should be used in middle of a line, not at the end of a line. Don't confuse them. Also, see rule #9.
Use dialog style as less as possible. Reason - Dialog style use more than 1 line with extra 2 dashes and 2 spaces, increasing characters to show as well as vertically consuming the space. Identifying who says which line is sometimes difficult to find in dialog style. Use of dialog style should be avoided as much as possible. Only 2 exceptions are: 1) more than 2 people speak simultaneously, and 2) more than 2 people speak shortly within a very short time of period.
Use numbers instead of spelling them as much as possible. Reason - Spelling out numbers as alphabet (i.e. three instead of 3, nine instead of 9) uses more space than just using numbers. For this reason, "3:00 A.M." should be "3 pm" and "at 16:00" should be "at 16." "7 hours" instead of "seven hours."
Use "..." (3 dots) whenever there is certain length of pause between words even it is in a middle of a short sentence. Over 0.5 second of silence is considered as a pause. Reason - Showing where a pause is one of very important information in subscript. It is essential part of play.
Use "--" (2 dashes) when a speech is interrupted. Don't use "..." for this. If not interrupted, just use normal period "." or other marks such as "!" and "?" Reason - Interruption and pause have each different meaning. When interrupted, there is no continuing part from the person. On the other hand, pause is always followed by continuing part. Don't confuse them.
Start with lowercase after the previous line ends with "..." except rule #12 case. Reason - Because the line is still in the middle of a sentence and there is no reason why to start with uppercase.
Use proper manner of upper case and lower case. Uppercase whenever appropriate and necessary. Reason - This is to identify proper nouns from other names, such as place, organization, person's name etc.
Acronyms are shown without periods and almost always in all uppercase. Reason - Omitting periods (instead of ordinary use) is to shorten a line as possible. That is, "F.B.I" should be just "FBI" and "C.O.D." (Cause Of Death) should be "COD." All uppercase is just a regular rule. Exceptions are, for example, PhD, IDs, iPad.
Song lyric is shown only when a character oneself sings. Theme song and other background song are skipped. Reason - Theme song is almost always the same. It is tiresome to include the same line each episode. If you are interested, just search for the song lyric. Background song often interferes character's talk. If try to include songs and a character speaks while playing, the song lyric becomes incomplete and the talk becomes much harder to read. Songs are just minor part of episode, focus on speech.
Don't use such characters as ":" and ";" unless absolutely necessary. Don't use foreign alphabet other than 26 A-Z such as "ü ä" in German, "ç à é" and French, and "ñ ¡" in Spanish. Reason - Because it is English subscript. That is what English means! Colon and semicolon are almost always can be replaced simply by period (.) or comma (,).
Always type as "Okay." "OK" "okay" "O.K." are not acceptable. Reason - "OK" is confusing as other possible acronyms. "O.K." is against rule #13. "okay" is wrong in that it starts with lower case.
Do not use a unit like $ unless it is actually said. Reason - Subscript is to help understand what is actually said, not to help understanding itself. If it is said "two thousand dollars," then it is "$2,000." But if it is said just "two thousand" and even if it is clear that it means "$2,000," it must be typed as just "2,000."
Do not use <i> and </i> to show in italic style or any other tags like coloring. Reason - Some subscripts use this type of tags to show that it is spoken by a person off-screen or on TV. Some even meticulously does this custom as soon as the scene is off a person just speaking. There is no clear rule to use this <i></i> tag in general. That confuses viewers. Essentially the tags are not necessary. What difference does it make to tell one on screen from another off screen? Just ridiculous.