I'm not very mathematical either so this may be totally wrong, but what about using one of those calories/activity calculators online and increasing you're weight by 75 lbs? That will increase your calories burned, and it might be somewhere in the ballpark. Then you can manually enter your calories burned on spark people.
Remember, probably neither those calculators, nor sparkpeople, take into account drag, rolling resistance, etc for just yourself and your bike.
But also, those speed ratings on sparkpeople are not accurate for the bike becuase they don't take into account your slope. Riding at 20 mph on a flat surface would be VERY different from averaging 20mph on a hilly course, the latter burning more cals. In fact, on a hilly course you'll almost always go slower, but you won't burn less calories than a course of the same length but flatter.