Piyo Piyo, if I may suggest something, try measuring the head clearance with a piece of solder. I find in Texas that .71mm to .76mm (or .028 to .030") is a good number for 30% nitro. That's usually .4mm thick head shims. I do not normally run 25% but I start on the high side to help keep from detonating.

You mentioned "I'm using 0.5mm clearance below the underhead". Does this mean .5mm head shim thickness? or .5mm head clearance? if .5mm head clearance, your shims are around .3mm which is probably too much compression. If you blow plugs like you describe, then it is probably detonating from overcompression.