Wouldn't it be less trouble to simply get the trip point value directly? (I took a meter reading with my Luna Pro the other morning -- I'm waiting for brightness that translates into 1 sec. @ f8 for my FP4). Knowing that, I will simply see what resistance I get from my photocell at that light level, and plan accordingly. A condensor may be necessary to amplify the energy delivered to the photocell, so that the value is more reliable; some testing should help me decide on that point.