I meant .2" which per my memory would be around 5 millemeters and/or .5 centimeters, please double check this.
I did a lazy "outside" measurement of the used belts (my bad), so take away a bit (the measurements S/B IC based).
This said, 21.2" would slip upon startup with minimul use, IMO.
Considering that your deck has already experienced stress/wear (pully replacement) I'll suggest you go with the upper range belt length mentioned (20.7").
I'll probably do the same the next time I purchase belts, even though the deck has low hours (considering its age of course).