A couple of points:

Yes, United's first offer is based on the length of your delay (unlike American, which seems generally to start at $300 regardless). Incidentally, that's the length of your delay to your destination, not just at your origination airport. For example, a year ago I bumped off the nonstop EWR-SFO. They re-routed me through Chicago 1½ hours later, with $100 credit (standard for up to two hour delay). So I wrote to UAL pointing out that because of the stopover I wouldn't get into SFO until nearly three hours later, and got a very nice letter back confirming that therefore my voucher should have been $200 (and including an additional $100).

In Chicago I bumped twice more ; the last time was a 3½-hour delay for $300 but I was able to stand-by for a flight that ended up leaving just 45 minutes after my (delayed) original flight.

Interestingly, in Hawaii last March I took a bump from the 11:00am flight to the 1:30pm for $200 (per standard compensation schedule). Then the 1:30 flight was oversold, but they were only offering $300...yet it would mean an 8½ hour delay onto a red-eye . So perhaps $300 is their maximum first offer domestically? Needless to say, I boarded my flight - I may be a mileage (voucher) whore, but I have my price!