3 * 60 * .3 == 54
Powers of 2: 1, 2, 4, 8, 16, 32, 64...

64 would probably be a good idea, but you'd get slightly less variance with 32.  Then again, since you don't get a share until around 50,000, there's no point in setting the local difficulty lower than you have to, unless you want the most accurate statistics possible... but then you can just use your mining software to see what your singles are actually doing.

Short answer: 64