Pump your tyre up until it compresses by 15% under your weight, for instance I have found that my 25c tyre needs around 85psi on my rear and 80psi on my front with my weight of 79kg. But without a gauge on my hand pump I just pump it up then sit on the bike and if it compresses too much I just add more air. I only know the psi because I have a track pump at home where I usually inflate my tyres.