this is done by making a rate one turn which is indicated by your T+B but not on your AH (Yes, I know you can work it out but really that's the last thing you want to be doing).
TAS/10 + 7 is accurate enough, and that's not very hard.
IMHO to preference the T&B over an AH in IMC would be madness.
Anyway, the concept is to scan all the instruments and process all the information.