I frequently use Rodinal (Can't find R09, which I liked very much once) at dilutions 1:200, 1:400 and 1:800 with FP4 Plus. 1:200 for 25- 30 mins; 1:400 for 65-75 mins and 1:800 can be practically left overnight. I use those solutions in order to achieve very soft negatives with good compensation and low graininess. However I found that the agitation has effect on the final result. Thus, with 1:400, vigorous agitation during the whole process will reduce the softening effect. I usually reduce the agitation period and increase the intervals as the development process reaches its end. For 1:400 i do NOT agitate after the half of the processing time. In general slow / medium films will have reduced graininess as lower contrast as dilution ratio increases. I cannot find really a difference between 1:25 and 1:50 for fast films.(posted 8306 days ago)1:800 with FP4 is ideal for overexposed (200 or more ASA) negatives, whit contrast images. The effect is a very fine and soft negative with nice mid-towns.
However, lower dolutions will reduce the edge effect.
Cheers,
Kiril