Here are two suggestions for finding your base:
My suggestion blatantly ripped off from Coach-to-the-Superstars Dave Salo is a 3 x 300 set described here -- U.S. Masters Swimming Discussion Forums
Mallory Mead's Open Water Thread (U.S. Masters Swimming Discussion Forums) defines a "Best Possible Interval" as the best interval you can hold for 10 x 100
