On the decline decline BP, I'm going to go off of the lower rep set given the higher you go in reps the further off you'll be when estimating your 1RM.
Now usually, I (personally) am good for 10lbs a push up to about 5 reps. I'm going to make a similar assumption here for you but also deliberately scale it back a bit to ensure we don't start you too high out of the gate.
Therefore, 265 x 6 = 325 in my way of doing things. This squares up almost exactly with your 1RM calculator estimation of 324lbs.
FINAL RECOMMENDATION: Base your loading pattern off of 320lbs to play it safe.
That should seal a 10lb PR of 335lbs on the decline and quite likely a bit more if we underestimated by 5lbs going into the loading pattern.
Just my $.02 but having done this before, it's the safe bet
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