Thanks for your answer Alan.

I had tried Thornton two baths with FP4+@100 and i like much more than with HP5+. The tones are nicer.
I still find more grain than with ID11, should i add more sodium sulfite like in D-23 formula ?

I had some uneven development on my negs if i don't agitate any of the two baths.

On the link you provided, it is said to change time in first bath only if i want to increase or decrease global contrast, it sounds logical, whereas Thornton suggested to put more or less sodium metaborate in second bath to achieve N-1 to N+1 development.
Have you guys tried one of these methods ? Do you have some times for iso 100 or 50 i should start from for my tests?