Of course you are right. However A1 gun immediately failed, being responsible of 8 out of 19 total failed shots. Other 8 failures, according to PoW GAR + Mr.Barben (Vickers-Armstrong) report are uniformly distributed in salvos from 11 to 18 (1 per salvo). The last 3 are not determined.Paul Mercer wrote: "...as PoW had 10 against 8 would it not be logical to assume that her rate of shells fired (until her guns started to play up) would initially be higher than Bismarck?"
Thus it's incorrect to think that PoW effective RoF declined significantly only toward the end of the engagement. Her output efficiency was always lower than Bismarck but not so much lower.
As you can see in the table below, showing average RoF values (meaningful or meaningless as anyone can consider them), the effective RoF of PoW was 1.4 salvos per minute while Bismarck's one was 1.6 salvos per minute. When adjusted by the fact that PoW had 10 guns vs 8, you get an effective number of shells delivered per minute that is 7.0 for PoW and 6.4 for Bismarck.
All these "effective" figures are independent from any "assumption" except the battle duration, that is clearly written in PG reports and that we can assume to be the same for Bismarck as well, except there are solid evidences showing the contrary.
I will not answer again to provocations like "flat earth theorist" as they speak for themselves, when someone is unable to counter arguments (e.g. Bismarck open and cease fire).