Clive,
when printing 8 x 10 from a 35mm neg and stop twice down on my 50mm 2,8 EL-Nikkor (ie. from f2.8 to 5.6) first of all sharpness and contrast are very good and provided the neg is normally exposed and developed, and not particularly thin I would probably have to expose for 15 - 20s, using Ilforrd MulTigrade MG RC paprer. My enlarger is a diffusor type.
peter