If you haven’t already, take a look at the Bwgen tutorial (written by philthemn), which discusses some settings that might be helpful.
As for using a delta setting to get into theta, I don’t think that would be very helpful. But I don’t know too much about this subject. What I gather, however, is that in general it is harder to entrain to a frequency that is very different from your starting state, so I would guess that aiming low would merely make it less effective, and might even give you a headache – again, I’ve only read a little, and experienced even less, so maybe someone more experienced will confirm or correct what I have said.