indiebob

Roll a d20 at the same time as you roll a d100 for the chance to hit or miss. If you roll a hit and roll a 1 with the d20 it's a critical (this will be exactly 5% chance, no need for rounding). If you roll a miss and roll a 20 with the d20, it's a fumble (you'd then roll the d20 again to decide the type of fumble). Or you could say that in either case it's a 20 to fumble or critical, depending on whether you hit or miss... mathematically it makes no difference.