Or, if you are not that much mathematically inclined, you could simply use something like Ilford EM10 exposure monitor (or a similar device).
Originally Posted by cornflower2
Nowadays you can find those Ilford devices, used, almost for free - I presume even "down under".... It s particularly suited for just this purpose, e.g. if you have a known good print on a certain enlargement and want to have the same amount of exposure on a different size of paper (i.e. change of enlarger head height). Much faster and less error-prone than fiddling with numbers in the darkroom. Also less prone to fogging paper than an electronic organizer/iphone/whatever screen.
Although, I do admit that I still use my Palm in the darkroom. However, when making prints or test strips, it's stowed away in the (light-proof) cupboard, and started using a foot switch: