Here's a new one to me after forty years in the darkroom. The water temp where I live now is at 80F. I have a lot of exposed 4x5 FP4+ film, which I will develop in Pyrocat-HD, using a 12-sheet Nikor tank (in numerous batches of course). I've done this successfully for several years, but the ambient water temp has never gone above 75 until now. So the calculations... show a dev time of 4'06". I wouldn't do that short a time in a tray, much less in a daylight tank with its long fill/dump times. So it occurs to me...

Can I use a higher-than-normal developer dilution to increase the development times to a reasonable, and repeatable, length?

My current dilution is 1:1:100, standard I guess. And of course it's important to keep the processing chemicals at close to the ambient wash water temperature. Chilling the chemistry is not really an option.
Is it feasible to go to, say, 1:1:200 and increase the dev time? I suppose there are two concerns here.
1) would there be enough developing agent to get to a normal printing density?
2) how would you calculate an increased time?
I have some exposed, non-project film that I had planned to use to calibrate the process before developing important film. But I wonder if anyone has tried this approach with any success? I don't want to spend half a box of film, and many scarce hours, testing this approach if I can pick your collective brains.
Ideas, anyone?