Because every lens is different regarding precise aperture opening, and every curve different with different developers, I nonetheless endeavored to do a test with my Toyo 45 with Ilford HP-5 in D-76. Tray processed. One sheet over the other very slowly. I will try Ilford DD-X developer next. Umbrella reading is f/22 throughout. Results? f/22 exposure should have been developed at 6.5 minutes, not 8: f/32 exposure is best looking negative at 9.5 minutes(n-1). f/45 exposure shy in the darkest of gray scale blacks but is still very nice (n-2). f/64 exposure falls off the scales. Perhaps HP-5 is best rated at ISO 800, therefore, for my camera. These are not prints. They are rephotographed negatives at same exposure without any photoshop correction aside from flipping the curve. Check attached file for a sense of these negs.