When I started to read about personal film speed, I found lots of websites showing detailed tests to do with a range of exposures and developing times.

Almost all of them came to the same conclusion - Halve the speed and decrease the development time by about 25%.

That was enough for me. Rather than do the tests myself, I just tried HP5+ at EI 200 and developed it for 75% of the ISO 400 time.

I liked the results so kept doing it.