The other thing to throw into this is, how accurate is your tyre gauge?
When I had a day job I had the means to check pressures up to 20 psi very accurately at home. One day I checked a brand new tyre gauge and it was over 3 psi out at 20 psi. Most of the few I checked were at least 1.5 psi out. Therefore I would suggest that if you set them to 20 psi on your gauge they cold be anywhere from 18.5 to 21.5 psi anyway and could be even further out than that.
I only tested gauge or plunger type gauges but digital gauges are only as good as the sensor and on a £5 gauge that is not going to be very good.
So I would say set them at what you are happy with, look out for uneven wear across the tread and adjust as required.
YMMV.
Cheers
John
