In classic lawyerese....it depends. 12.L is your overall guide. Because you said the total was 21:00, it would seem that your total credit is based on the average daily guarantee for a 4 day trip. In that case, merely blocking one hour over scheduled would not increase the total credit as you are merely eating into your ADG.

If,.for example, the 2 hour difference (19:00 BL/21:00 TTL) was solely based upon 2:00 of rotation credit (see 12.K), then you would have to overblock by 2 hours and 1 minute to see the total credit increase past 21:00 (same as ADG)....unless your overblock is on the last leg and extends you past the scheduled block-in time of the original rotation whereby you will kick in rotation credit (1:3.5 as described in 12.K) to the tune of 1 minute for every 3 1/2 minutes you exceed the scheduled block-in for the last leg.

There are numerous provisions that can account for the 2 hour difference (ADG, deadhead legs, 1:2 duty rigs) so it is a hard question to answer without seeing the actual rotation. However, a 21:00 hour rotation is likely based on ADG as mentioned above.

Clear as mud?