I just do it - intuively grab a lens, compose the shot, and if it's fishy either try another
lens or give up. I'm very, very nickpicky about composition; and if it doesn't look right
on the groundglass, I don't know what good some handheld cropping gadget will do.
This doesn't mean my final shot is exactly what I at first imagined, however - I often
readjust the camera and explore around through the ground glass itself before reaching
for film. Might take 30 seconds, or might take half an hour!