[QUOTE=Kruger;1529879]@ thegman, thanks for the explanation. But then why do they recommend 4 stops to this other guy which had the same problem (100iso exposed at 1000 iso). What do you think ?

100 to 1000 is "3 and bit stops", exactly three stops is ISO 800, and exactly 4 stops is ISO 1600. Whether you do a 3 stop push or a 4 stop push depends on whether you'd rather a little under exposed, or a little over exposed. It's really just a matter of preference. Personally I'd go 3 simply because it's less of a push, and also slide film is often more comfortable with under exposure rather than over.

But it really is just a personal choice.