I agree with KSH and go with the times she suggested. In June I did 2 pool swims (my first in tris). The first one I put the time down that I knew I swam- and had to pass 6 people (5 in a row). The second time I put down a little faster time and didn't have to pass anyone. I'd make it a little faster than you can do right now.
I like the woman who said cluelessly at the first pool swim tri I did "oh... is 6 min fast for a 500???" DUH! That's olympic fast, stupid lady! I wonder how many people had to pass her? Sheesh...
She wasn't trying to cheat the system- she just had no clue. I think some people are that way when estimating their times.