Hi there Will
It's a bit 'buried' but follow this link
https://www.duckhams.com/zddp-what-does-it-all-mean/ and look down the article to the 3rd paragraph before the chemical formula. This refers to 0.12% (i.e. 1200ppm) maximum ZDDP for API SH implying that this is around the 'optimum' level in Duckhams. The product spec is here
https://www.duckhams.com/wp-content/uploads/2017/11/PDS-20W-50.pdf. I don't know why they don't spell out the ZDDP level more clearly. It could be a selling point. Using the term 'maximum' is also not helpful although correct in the context of the API standard.
I contacted Duckhams to confirm what this means precisely, e.g. mean and range or other statistical measure.
I'll post their reply when I get it.
Yoom a proper bey then. See you dreckly.
Andy