go to src/map/status.cpp and search case SC_CONCENTRATE: then if you find this line
case SC_CONCENTRATE:
val2 = 2 + val1;
if (sd) { // Store the card-bonus data that should not count in the %
val3 = sd->indexed_bonus.param_bonus[1]; // Agi
val4 = sd->indexed_bonus.param_bonus[4]; // Dex
} else
val3 = val4 = 0;
break;
change value 2 to your desired number. Need remember this skill upgrades your agi/dex by one per level, if you put 8, at max level you'll have +18 AGI/DEX