LBA means (+/-) : "where is the 1st block where my data are ?"
When you binhack a 1st_read.bin, you tell your DC where it must look for the session where your data are.
So, you're right.
If your 1st session is bigger than a 3 seconds audio file (for example, some CDDA), your LBA will be greater than 11700, since your data session will be after that audio session.
So you're still right.
When you burn a dummy file, the only thing it does is to move all the other files to the outter part of your CD. Since this part goes faster than the inner part of your CD, data are read faster.
But since you burn your dummy file as a data file, you don't change the 1st session length, so you don't change the LBA value of your 2nd session.
Read this :
http://www.megagames.com/dc/dc_backup_faq.shtml#3.7