There seems to be a rule about 30 minutes. No matter how early the previous flight arrives, they will not do anything until exactly 30 minutes. If you preboard as do I, then be there at 30 minutes.
Depending on the number of preboarders, it can be 0-10 minutes before BS starts. I have seen flights where I was the only preboarder, I walked down the ramp at T-30 and was followed immediately by BS. If you are in the A group, 20 minutes may be too late. If you are in C, then just be sure to get there at T-10+ or they may give away your seat at T-10.