f8 on an EL-Nikkor 50mm, but for no terribly scientific reason! It gives me useful exposures of between 10 and 20 seconds for most straight 10x8's on Ilford MG WT RC and I have no gripes about sharpness or distortion.
Allow me to tempt you spend money, however:
I bought one of these late last year after humming and hawing about it for a longish time because of the undeniably steep price (about 2/3rds what I paid for the rest of my darkroom!. Best money I ever spent. My only regret is that I waited so long.
I printed a B&W calendar for family and friends last year made up of, 12 5x7's mounted on a pre-printed calendar. I did 9 of the *%$&**$% things and if it hadn't been for this timer I'd be printing them yet!
Sorry to wax lyrical but really good kit brings out the evangelist in me! ;-)