How do I figure out how much more insulation I need to hit R-60?

The varying depths you measured are totally normal, by the way. Old blown-in settles unevenly, and whoever did the original job probaly just eyeballed it too. Don't stress about the 5 vs 9 inch difference, but do take a bunch more measurements and average them out, because that average is your starting point for the math.

On the raking question, yes, spread the thick spots out before you blow anything. I used a garden leaf rake and it worked fine, the old stuff pulls around easier than you'd think. For hitting your depth, what saved me was cutting a few scrap 1x scraps marked with lines every inch and poking them down at random spots as I went. Blown insulation fluffs up more than the bag math suggests, so trust the ruler over the coverage chart. Also blow perpendicular to the joists at the end to knock down the ridges.

Air sealing is the part people rush and it's honestly more work than the blowing. When I did mine I spent a whole Saturday crawling around with cans of foam and caulk sealing every wire hole and the tops of the interior walls. Stuffed the big chase openings with cardboard first, foam over that. Also make sure you keep an eye out for old knob and tube wiring if the house is older, burying that hot in insulation is a real problem. One more thing, throw a couple rafter baffles in at the soffit ends before you blow. Way easier to staple them in while you can still see the edges than trying to fish around after the attic is deep in fluff.