any clock has "f-stop" capability with a "Brain 1.1" upgrade - you! ;-).
The other advice isnīt helpful, either - some time ago I found a cheap GDR darkroom timer that has this function. RTF Exakt, build in mid1990 just before the company collapsed... I doubt it was exported nor build in high numbers the price in 1990 was 195Mark, quite steep. Got if for a mere single Euro - it was a blind buy at Ebay.de, the seller didnīt know what he was doing and I had a slight suspect about itīs functionality. ;-)
Perhaps some of you stumble across one - small, bright orange plastics. If so - grab it and run.