Wes,

I used average time of winners over a large numer of races.

I think if you are going to use time ratings you have to do them properly. i.e you need track variants and ideally you would also apply a daily track variant to account for track condition, rail position and wind (I'm not doing this - too hard)

I used track variants and there can be more than 1 sec variation over 1200m from one track to another which makes a big difference.