D&D 3.5 - Crit Damage Macro

So, our DM has her own method of critical damage on a Nat 20, and I am trying to find a universal way to make it work in our macros. If weapon damage is 1d8, i want it to take the max number possible, which is 8, but if another sheet has 1d10, take that as a 10 instead, is that possible? Thanks for the help.

You should be able to just change the crit calculation to manually override the normal crit rules: from  [[ (@{weapon1critmult}-1) ]] d6 + [[ (@{weapon1critmult}-1) ]] *( @{weapon1damagestat}[Weapon Dmg Ability] +@{weapon1enh}[Weapon Enh] +@{weapon1specialize}[Weapon Spec] +?{Power Attack Bonus?|0}[Pwr Attk] +?{Additional Damage Bonus?|0}[Ad'l Dmg Bon])  to   6 + [[ (@{weapon1critmult} -2 ) ]] d6 + [[ (@{weapon1critmult}-1) ]] *( @{weapon1damagestat}[Weapon Dmg Ability] +@{weapon1enh}[Weapon Enh] +@{weapon1specialize}[Weapon Spec] +?{Power Attack Bonus?|0}[Pwr Attk] +?{Additional Damage Bonus?|0}[Ad'l Dmg Bon])  the areas in bold will be custom per weapon of course but that should do it, note how the critmultiplier for the dice changes to -2 (italicised for your convenience) since we are assuming the first die is maxed... this means for a normal x2 weapon you'll just get the flat 6 on top of your normal damage, for a x3 weapon you'd get the flat 6 + 1d6 (then the rest of the crit stuff too). Hope that makes sense and does what you need, been a while since I dabbled in 3.5e and already I have a headache from it :D