Look for an old Kodak Dataguide calculator
It has a wheel type claculator - one part stays still, and shows temperature through a window in the part of the dial that moves. The edge of the moving window is calibrated in minutes (or I might have that reversed) On the bottom of the dial that moves there is a pointer scale with calibrations that goes something like 50% through to 200% of the normal time mark (which falls at 100%) The pointer matches up against a number scale that I remember as being called a development number.
I mix my diluted developer - rarely is it spot on 68F/20C. I know from past experience /testing/ dataguide recommended starting point now refined, that say, 35mm TriX D76 1:1 is Development Number 39 to give the best gamma to print mid multigrade on my diffusion enlarger. So I place the development 100% mark opposite 39. The window now shows the time temperature options available to me. I look at the thermometer, and pick off the time to put on the old Gralab 300, pour the solution in, and start the timer.
It should be possible to finf other sorces to make your own dial or sliding scale. I recall that Ilford in their published data showed the curves for times at different temperatures- that will guve you the answer as well. Locate the time for any developer at the 20C line, then follow the curve that intersects your time at 20C, to find the appropriate non 20C time.