You may be sometimes close with that 30%. Not trying to sway you either way, but one of the USP specifications applicable to these products is potency and the specification allows for 90-110% - a potential 20% difference between 2 products. It isn’t going to be 1:1 with regard to difference in potency and difference in lab results because there are many other random factors.
To be clear, both compounders and the big pharma companies must adhere to the USP specifications. It is not uncommon to get a medication from a compounding pharmacy, Publix, or CVS that is not exactly 100% of the strength stated. Some medications have even greater variances permitted of +/- 20% if you can imagine that, and some less at +/- 5%.