I'm not even a chemists assistant. I know nothing. Here's what you do;
Search [whatever precursor] mol.wgt ( molecular weight)
2-bromo-3,4-methylproprophenone mol.weight=257.1grams in a mol.
Search for your target chemical mol.wt
Beta-keto methylenedioxymethamphetamine mole weight =227.23g/mol( I rounded)
For every 257.1g of 2-d-3,4-mdp there is (in theory)227.23g of bk-mdma
227.23g/257.1=0.88381, 88%
That's theoretical perfect quantitative yield. If people are getting 80% yield/ you hope to get 80%, then you multiply that by 0.8
88.381x0.8=70.74%
If 1kg is 70.74%, 1/70.74x10=1.292
1.292kg of 2-b-md-p to make 1 kg of bk-mdma
The yield I didn't look up, I just used a number that sounded reasonable (70%) this is for illustration purposes only. Now you don't have to look like an idiot who couldn't pass the 6th grade.