You can see a discussion of the 15% tire drop inflation recommendation here.

That article only discusses tires up to 37 mm wide. I put together a spreadsheet that extrapolates the tire size up to 60 mm. The coefficients for 18 to 37 mm tires follow smooth curves so the extrapolation is probably reasonable, just be aware that it is not based on data for tires larger than 37 mm. I think the principle here is that less than maximum pressure in a tire will often result in a smoother ride with nearly no penalty in rolling resistance. On rough roads a lower pressure can reduce the rolling resistance in fact. But the pressures recommended in the article or my spreadsheet are not Gospel, they are good places to start and you might want to use higher or lower pressures once you have tried them.

The system here does not allow me to post the spreadsheet but I can post images of the charts the spreadsheet can generate.

KenBertoWideEng..pdfBertoWideMet..pdfBertoWideMix..pdf