I use rodinal 1:25 with FP4+ all the time at around 70degrees and i have no idea what grain you are talking about
I once spent a good 10 mins trying to focus a fp4+ negative on the enlarger cause the grain focuser didnt show any grain! so i went with the sharpness.

I just hate staying in the dark for a long time so i always use 1:25 with rodinal. 1:50 or 1:100 would take forever.