There was a couple months in 2015, where there was such a shortage of captains that the some of the more senior CQFOs were awarded captain reserve lines for the month. In that case, they got guarantee at the captain rate, instead of the CQFOs blended rate for the entire month.
I don't foresee that we will get that short on captains again any time in the near future.
Realistically, most of your pay is going to be at the blended rate which is about $51/hour currently. Occasionally you will get a captain trip, and that will be paid at full captain pay.