OK, the lower front yard lawn has been sinking the past 20 years. It was originally an erosion gully and I had it filled 20 years ago. I had an entire dumptruck of regular yard soil added there and I spent a week spreading it out. But it has sunk over the years and any rain collects in there; its a swamp now. I dug a trench from there to the street drain, but it always fills up with tree debris and gets blocked.
I need more soil. But how much? Well, I'm going to set up stakes with strings on them at the level that seems best. I'll measure severel places under the strings to estimate the soil needed. I am good at geometry. Then I'll add 50% for packing the soil down a bit. Not too packed, but it has to be packed enough to resist just being washed away in storms until new grass grows. And a bit too high would be better than too low. After all, too low IS the problem.
I'm tired of half the front lawn being squishy to walk on for days after every rain (and unmowable). Last year, I thought the drainage ditch I dug would solve the problem. It doesn't. Time for a more permanent solution.