In my experience 15 minutes is longer than necessary; I stick to 11 minutes max unless I've exposed at a higher EI.
But since you are using sheet film it will be relatively easy to dial it in. Make a few identical exposures and develop one sheet at 11 with reduced agitation (30 sec to start, then gentle agitation every 3 minutes) and see what you get. If you end up with an optimal negative, then no more testing is required unless you want to see the effects of different agitation regimens. Or you could try Rodinal 1:25 (5-6 minutes) and 1:100 (~19 minutes) and compare.
Just my deux centimes.