FP4+ in perceptol 1:3 question

I usually develop HP5+ in 1:3 perceptol using a jobo processor in a series 3000 drum. The results are as expected. Today i developed FP4+ exposed at 64iso for 17 minutes and the negatives are nearly see through and this was on a sunny day exposing for the shadows.

Anyone have any ideas what could be going on ? I use 500ml of working solution which comes out to 125ml of perceptol plus water for 10 sheets of 4x5 that fit into the tank. Could it be that the chemical became exhausted ? It does not seem to when used with HP5.

What is the best way to test developer + film combinations to get predictable negs ?

10 sheets of 4 x 5 in 1+3 Petceptol with only 125 ml of developer?

I think that may be pushing it and the developer got exhausted.

I used Perceptol 1+3 for a while but only for 120 and usually only 1 roll at a time.

I've been having amazing results lately with this film and hc-110, dilution B (seasoned developer) for 7-9 minutes depending on lighting conditions....just sayin.

But to answer your question, the "best" way (in my opnion) is to take a white towel out on a sunny day and place it in a single situation where the towel is half in the sun and half in the shadows and expose for the shadows and develop for the highlights. Shoot an entire roll and just do a snip test development for 1/5 of the roll at a time. Start at the "lowest" plausible time of development in increments of 1,2,3 or 4 minutes (depending on your developer strength. You can then take the negs and analyze which are the best (in my opinion would have very light textured highlights in the white that is in the sun, but also retain "good" shadow detail) and voila...you have a personal ISO for that film and developer combination.

at 17 minutes though, I wouldn't imagine your negs would be thin. You could just run a roll of fp4 with your hp5 and see what happens?