as mentioned in the op, i'm not 100% sure how to do that.
in the op i suggested just taking the hc rating and subtracting 1 to 1.5 letter grades (i.e. and A+ hc would have B or B+ oc/dc). but once you got down to the D+ rated coaches it wouldn't work much.
and, of course, for the higher regarded oc/dc, maybe bump them up another +/- rating. like malzahn at auburn. chizick is rated a B+ by the system (which is fair, imo). but malzahn is certainly not a C or C- and probably better than C+. so i'd probably bump him to B-.
same with kirby smart at bama (though his rating based from his hc (saban) would be B or B+, so i probably wouldn't change that one).
likewise, for coaches you feel are not very good, drop them abother +/_ point.
the key here is to rate them as if they had just taken over as a hc, NOT how they are as an oc/dc. cause, while malzahn and smart are 2 of the hottest oc/dc in real life and probably be rated A or A+ as oc/dc, neither would be in the top 25 hc and neither would be rated above B or B+ until they proved it.
and since there is no drop in the game for going from oc/dc to a hc, you have to account for it up front. (note, once in dynasty, since we can't edit, a couple years down the road this will not be as effective. but initially, it is the best way to go forward, imo)
another thought, and this would take a lot of work so i'm not sure i'm up for it, but it would be to take the points for (for oc) and points against (for dc) and do a rating system like above for hc's.
biggest problem with that is that oc/dc are always associated with those hc. in fact, most have only been with their current hc for 2-3 years. and i can only get hc #'s for points for/against. so using those #'s would be not entirely accurate. not accurate at all, really.
so that one is not likely to happen.
got any suggestions?