I have a stash of tech pan, and have calibrated it with a mix yourself formula in the 'Darkroom Cookbook'. I was lucky enough to buy an old unopenned 8x10 box of 25 sheets for $4 at a swap show. One corner has uneven emulsion from pressure on the box for a number of years. I cut the 8x10 down to 4x5, and use the screwed up corner to calibrate the developer for N, and N-1 development so far.
Now I can use this developer with confidence on the 3 rolls of 120, and 2-rolls of 35mm TP O have in my frozen horde. The challenge now is to actually use the last of the stuff up.