How to properly incubate soft-shell turtle eggs?

Ok. My husband and I caught 8 softshell turtles. He killed them as he cleaned them. Ex- kill one clean it, kill another clean it. So they weren't left to ruin or anything. Anyway. 4 of them were females with fertilized eggs. So we kept the eggs (65 in all) and I have them under a lamp with damp sand on bottom and an aquarium heater in the sand that turns itself off when it reaches 80 degrees F. Our local pet store said anywhere between 75-90 is ok. And that is exactly what is going on. I can't find the perfect spot for the lamp the middle eggs are a tad too warm and the ones outside are a tad cool. I'm trying my best. But the lamp is a purple night heat bulb. Not sure wattage as we had to make do with what we had. Any suggestions? I didn't take the eggs from a nest. We took them directly from the turtle. And they have hardened and I'm assuming the air pocket in the egg is the white spot? I was trying to see how warm the egg was and that part of the shell got stuck to my thermometer =( I put a bandaid on the hole immediately. Not that it will actually work. But I'll try anything to save them. Help plz...