Of course when using the Tas Tab ratings, you have to decide which race or races you use as your base rating run. That is very important. You could just simply use the last run, you could use the best of it's last 2 runs, or the best rating recent run over this distance. You could get an average of 2 of it's last 3 runs. It's up to you what you do, but once you've got the Tas Tab Rtg that you want to use for the horse, you do this:

(Tas Tab Rtg divided by 10) + (Unitab Rtg divided by 2) - Weight to be carried.

That's a very simple version. Personally, I also add a few small bonuses and penalties to this, but generally, there's no reason why you couldn't just use the simple version.