Just changing to the correct steps/mm fixed it. I didn't have to do anything with H values as my machine was pretty structurally accurate, and I didn't have any noticible effector tilt. It's printing pretty well without me doing any additional tweaking.
When I thought about it some more it made sense - if the carriages aren't where they're supposed to be due to the wrong steps/mm, there will be more or less travel for each carriage required to lower the probe down to the point where it triggers. I stumbled onto it because I was checking everything to do with how I set up the stepper drivers - 80 steps/mm was the default and I just never changed it. I used the stepper calculator here: http://prusaprinters.org/calculator/ and put in my belt (GT2) and 16 tooth pulley, it said 100, I swapped that into my config file, and it now probes correctly. Double check your pulley and belt info with that calculator - it might be your issue too.
