engine compression (psi) and compression ratio are almost completely unrelated. depending on exhaust port duration and positioning, engine compression (psi) values can vary greatly on the same cylinder/head combination. dont count on compression (psi) values to determine if you're compression ratio is high.
with that said however, milling your head will give an increased engine compression AND compression ratio if the cylinder port timings are unchanged. so in this case your CR has increased leading to detonation. i would probably just go up in octane ratings until your detonation is inaudible, then perhaps up another octane rating once you stop hearing the pings just to be safe (at the expense of small loses in power because of unburned fuel).