It has taken me years to realise that the asa speed quoted on the manufacturers box is not necessarily the correct EI (Exposure Index) for my own particular style of usage. Only by carrying out personal tests can we arrive at the best EI for us. I will if I may give my own EI examples. Maybe other may like to add theirs?

Ilford Pan-F-Plus box speed 50asa my EI 40asa
Rollei Retro 100 box speed 100asa my EI 50/80asa
Ilpord FP4- Plus box speed 125asa my EI 100asa
Ilford HP5-Plus box speed 400asa my EI 300asa
Kodak Tri-X 400 box speed 400asa my EI 320asa
Kodak Tmax 3200 box speed 3200asa my EI 1600

All above EI are processed at recommended times other than Tmax which I reduce by 10%. It would be fair to say that the above might be applied to all film box speed i.e. reduce all film stock box speed by about 20% but the only way to be sure is to carry out personal EI tests.

Can I say that I highly recommend pursuing this line of approach as it makes amasing effect and dramatic results on exposed film stock

Great subject! As both Kenneth and D Max wrote, it is highly subjective and tied to your preference and equipment. To be able to vary the EI and adjusting for it during development is the main reason I started developing film myself.

As mentioned above, you need to develop a feel for the impact of these changes yourself. I did get some great results with IIlford FP4+ at ISO 200 as I wanted to get a very hard, film noire result which I like for cityscapes (http://monochrome.me.uk/blog/browse/tag/noteAE/1) . At other times I have shot FP4+ at 100 and 80 ISO. So it is not just a case of lowering the boxspeed at bit.

I mainly vary the EI based on lighting conditions: if the scene has little or no contrast, I lower the ISO setting on my camera a bit to overexpose 1/3 or 2/3 of a stop and if there is too much contrast on a sunny day in the middle of the afternoon, I go the other way. Shooting medium format with only 10 or 16 exposures on a film does help as it is easy to finish the film, note the development recommendations on the paper on the outside of the film and load a new film for different lighting situations.

If I know I want to stand develop the film, I usually shoot at a higher EI to avoid blowing out highlights because of this development method.

I do take the film I’m using into account, for example, ADOX CHS 25, my favourite film, cannot be overexposed and I always shoot it on boxspeed even when stand developing.