development times:condenser vs. diffusion

Kodak tables of development times appear to be for diffusion enlargers. They state that development times for condenser enlargers need to be reduced by 10-20%. Questions: - why different time for different light sources? - 20% (Ithink) is 0.5 to 1 stop; is this really important - the difference maybe corrected in printing? - when I do reduce times by 15% (tmax/D76(1:1) and tmax/xtol(1:2)) the negatives are light - what gives? thanks in advance Jim Hanson

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