Hi Dwane Dibblie.
As the armour is far more resiliant than the creature behind it in 40k type games.(Usualy.)
The the chance of a creature suriving a hit that beat the armour is not that great.(When compared to games like WHFB,for example.Low velocity weapons like arrows and swords are less truamatic than highvelocity weapons like HVAP and Plasma!))
So if we let the toughness value modify the armour value , and deduct this value from the strenght of the weapon to get the armour save.
It does exactly the same as you sugest in your post , but arrives thier by a more straight forward way.
1)T vs S = modifier.
2)SV - modifier= modified save.
OR str- (Tm+ sv) = modified save required.
Using current 40k values....
T up to 3 no modifier.
T 4 to 5 + 1
T 6 to 7 +2
T 8+ , +3.
6+ save = AR 1
5+ save = AR 2
4+ save = AR3
3+ save = AR4
2+ save + AR 5
Power field modifier to save.
Refractor. +1,
Deflector.+2
Displacer.+3
Current vehicle AV + AR.
This way ALL units get an AR value from 1 to 14.
Current weapon strenghts +3 =New 'Damage Value'.
Eg Ork Boy (sv 6+ T 4) AR 2 is hit by a bolt gun (Str 4 + 3 ) Damage 7
6-2 = 5+ save.
(If save is failed the model is removed as a casualty.)
An SM (sv3 T4) AR 5 is hit by a bolt gun (Str 4 +3) Damage 7
7-5=2+ save
(Values in parenthasis for reference only.)
This way a models armour and toughness MODIFY the damage taken directly, to give proportional saves....
And as this is so simple it alows for a supression mechanic and detailed 'large model damage' to be resolved using less pages of rules than the current 40k rules use JUST for resolving damage.
(Monsters and vehicles are treated the same in my concept.)
I may not have explained it that well sorry...
TTFN