How much chilli puree should I add to get 43 ppm pungency?

Unless the chili puree you are using is an industrial product that comes with a narrow and clear specification (in SHU or ppm capsaicin), OR unless you are capable of measuring the SHU or capsaicin content yourself, this is not possible to do with any degree of accuracy - if you are processing fresh or dried chili yourself, the actual content is very dependent on natural variation and your process.

Also, these scales (ASTA ppm vs SHU) only roughly translate because of weight vs volume vs dry mass vs ... issues, and because SHU itself is not considered a truly accurate measurement.

Naively calculated (this means: if you need a plausible and demonstrably calculated BS result because somebody needs some number on some piece of paper, here is how you get your BS result), 16.000.000 SHU would be a (non*)food consisting of 100% capsaicin, so 1000ppm would be equivalent to 16000 SHU, 1ppm to 16 SHU, 43ppm to 688 SHU. If your capsaicin-bearing ingredient has eg 6880 SHU (not unrealistic!), you would use 10%.

*nonfood indeed. Pure capsaicin, while perfectly safe when incorporated into food in sufficient dilution, would be very harmful to ingest on its own!