HI Laura
It really doesnt matter what level you send tone at. What IS important is the relationship between that tone and the peak of the audio that follows it. That relationship has been worked into an industry recognised standard and therefore its easiest to follow that standard.
In the UK it is 1k tone at -18dbfs = PPM 4. The peak audio level is then +8 db above that. But to re iterate it is a standard that people follow and take for granted, but you could pefectly correctly put tone at -14 to PPM 5 as long as you document this on the tape report that this is the rule that you are adopting for delivery so the receiving engineer could calibrate to that.
I dont really know what the US standards are, but all this spec will be documented on the website tech area of whoever the ntaional broadcasting governing body is in your country. In the UK its the EBU (European Broadcasting Union).
The same rules apply to the bars that preceed delivery copy. The Bars can be 75% or 100% and it doesnt matter as long as you document what is usd.
Peter
Dubbingmixer