I use something like this on the rare occasion I venture into memory:
Welcome to Corsair :: Memory Configurator
I too used to know it, but the numbers were smaller - I always read that the pairs should match, and that the difference between one 8GB and 2x4GB is pretty much negligble, unless you're tearing the arse out of graphic manipulation and even then that'll depend on whether the MB has a choke point somewhere else (is it really dual channelled all the way). Here, for example, the same arguments are trotted out as have been since the days of the PII
https://linustechtips.com/main/topic...ich-is-better/ 
I tend towards the "2x doesn't give you advantage, value upgradability more.