Hello to all of you out there!

I am new to making soft (on/off) subtitles. I don't really know how it works. I will use I-Author to make the soft subtitles but I wanted to know before hand, that if I make the subtitles for my creations, would the files size gets larger? If so, how would I be able to figure out what bitrate to use to max my cd? Right now, without multi-audio, subtitles, etc. I use the bitrate calculator to calculate my bitrate. And so far so good. It does its job and I was able to fit a 817 MB into a 80 min disc without over-burn. So, my main concern here is, if I added the subtitles to the file, obviously the file will be larger (but the lenght of the movie doesn't change and the bitrate cal. uses the lenght of movie to calculate). Would I still be able to get an accurate bitrate? Thanks for all your attention. Jordan