Yes, that’s similar to what I described I think. But suppose you have a roof that is 12.2 m long. You want rafters at 400mm spacings. Now I have to do some mental arithmetic: 12200/400 = 30.5. To get regular spacings, I need to round up to 31. Now I can copy and divide by 31. Not so hard to do but if you are doing a number of such calculations in quick succession it does take time and can be prone to arithmetical error. I was just looking for a way of speeding it up.