You may find you need to rate FP4 slower than 125. I prefer EI 64 in my RF645 and Nikon F100. Exposed this way I develop in ID-11 1:1 for 10 minutes. Very nice results. I would stick with the 1:1 dilution and use the developer one shot. 1:1 also offers more sharpness than undiluted and also doesn't require long development times like 1:3.
You're going to have to run tests. My recommendation is to shoot EI 64 in bright sunlight and develop 10 minutes 1:1, and EI 100 or 125 in overcast light and develop 1:1 for 13 minutes.