I have 28 f & 30 rear, for 80% bitumen 20% dirt roads - great on the black top and acceptable on the dirt.
But my bike weighs about 330 kg with a full tank and me aboard, and I dont push things on the dirt.
I would start there , then try reducing the pressures a couple of lbs at a time until you find something that suits you.
Adelaide Hills, Australia. 93 R100 GS, 77 R75/7 ,70 BSA B44VS, , 86 R80 G/S PD, 95 BMW Funduro F650 ST