We consider effective radius in brake Calcs Rich...
Diameter is only here to sell kits
For example
TTS disc is 340mm, R32 disc is 345mm. Pad sits in exactly the same position on both is the R32 disc better even though the extra 2.5mm is just creating a lip and isn't touched by the pad???
Also you would normally use the centre point of the pad radially for calculations, some pads are available with hugely different heights from 41-54mm say.......so by using a smaller pad you are technically moving it further out...that make sense?