Slide copying advice

I'm going to be copying some slides onto black and white film. Preliminary tests have proved (not unexpectedly) somewhat contrasty and lacking a bit in the shadow department despite my downrating the film (HP5) by a stop. I've decided next time to try some FP4 downrated to 50 or maybe even 25 asa and pull development by 30~40%. What is the forum's advice on this. Many thanks, Blights.

Long ago I pulled other 35mm B&W films about 30% or 40% when copying slides. Since I bracketed when doing this, I didn't make note of the exposure compensation. Several stops sounds about right.