So here is a quick sum to use to help you work it out.
For example. I want to work out the weight of a 16pp A4 brochure which has a 4pp 350gsm cover and a 12pp 170gsm text.
A4 size is 297 x 210.
Cover = 0.297 x 0.210 x 350 (grammage of the paper) =21.83gsm
Cover is 4pp so that is 2 leaves so time by 2 – 21.83 x 2 = 43.66
Text = 0.297 x 0.210 x 170 (grammage of the paper) = 10.60gsm
Text is 12pp so that is 6 leaves so times this weight by 6 – 10.6 x 6 = 63.62
TOTAL – 42.76 + 63.62 = 107.28gsm per brochure
In his own words here is Clive describing the workings out in a short video
Simple as that!