What you are looking for is an exposure that leaves 20 and 21 clear. This allows you to measure the effects of extended development on the base+fog, which affects your film speed.
Have you got Plotter yet? There's a demo available. I was a little apprehensive about doing this but I feel now that it will be of great benefit. Incident metering is my preferred way of working, and I shoot a lot of portraits in open shade, so being able to accurately expose these and develop them to the right values should speed things up in the darkroom.