Hey, sorry to necro a dead thread, but I actually set up some custom zones for 5K and Marathon pace based on the Running Writings calc. You have to do some math to get the percentages correct, since as you point out the pace percentages are calculated differently, but I did it.
The two zone sets are “Marathon Pace Zones” and “5K Pace Zones”, and they depend on two fields, “Marathon Pace” and “5K Pace”. These are public – if you search for them, you should find them
I added most of the breakdowns he uses, as well as a couple of others. For marathon pace, I had to get more nitty gritty, since he uses 90, 92, 94, 95, and 96, so the ranges get a little tight in that area.
In general, I would either do the lower bound to upper bound (e.g. Easy is 65%-75%) or a little around each, (e.g., 95% is actually 94.5% to 95.5%, and 96% is actually 95.5% to 96.5%, I think)
EDIT: A little more detail about the math/calculations in this thread, if you want to reverse engineer exactly where the cutoffs are: Custom zones work started - #115 by Pierre_du_Pont
