The green cells below allows entry of degrees of temperature (column C), a choice of C or F temperatures (D2) and a standard time for development (e.g. from the massive development chart at ).
This then calculates the adjusted time for the new temperature – NOTE: THE RESULT IS AN APPROXIMATE VALUE.

This works in both “Open office” and MS Excel…

  A B C D E
1 Farenheit k -0.045        
2 Celcius k -0.081 Use: Celcius k -0.081  
4 Known Temp 20.00 Time 0:09:30  
5 New Temp 20.50 Time 0:09:10  

EDIT: Added “0:” before the times, which stands for HOURS.
If this isn’t there you’ll get confused with hours being the leftmost digit.
Also: The Celcius k should read -0.081.

Ilford has a temp chart that differs slightly from the result of this, use -0.051 and -0.090 as k’s for results closer to that table.

Klick on cell E5, and then type:

In cell E2 I have typed
… but it could just as well be the value for Celcius (or Farenheit) from B1 or B2.

To continue my style; click on cell D2 and select “Menu: Data > Validity… > Criteria” and then enter the settings:
Allow; Cell range
[v] Allow blank cells
[v] Show Selection list
[ ] Sort entries ascending
Source; A1:A2
Click ok [OK]

NOTE: There is a similar setting in MS Excel, I know because I have used it, but cannot check the exact steps as I haven’t got access to it at home

Now, when you can click on D2, you can select Celcius or Farenheit – by that setting the value in E2, and affecting the calculation accordingly.