Ah I see.
Wouldn't a better solution be to reduce the base true damage on Noxian Guillotine, but make it scale more off total AD? This would be a nerf to Full Tank Darius, but a buff to glass cannon / high AD build Dariuses, and neutral impact to a standard build. Example:
Current R1 (@ level 6): Deals 100 (+ 75% bonus AD) true damage
Proposed R1 (@ level 6): Deals 40 (+ 75% Attack Damage) true damage
At level 6 with Black Cleaver he would have 81 base AD + 50 AD (from BC) = 131 AD.
Current R1 deals 100+37.5 = 137.5 damage
Proposed R1 deals 40+98.25 = 138.25 damage
At level 6 with Sterak's Gage (instead of Black Cleaver) he would have 81 base AD + 40.5 added base AD (from SG) = 121.5 AD.
Current R1 deals 100+0 = 100 damage
Proposed R1 deals 40+91.125 = 131.125 damage
At level 18 with BC+SG+TH for damage he would have 141 base AD + 50 (BC) + 70.5 (SG) + 35 (TH) = 296.5 AD
Current R3 (@ L18) deals 300 + 75% bonus AD (63.75 from BC+TH) = 363.75 damage
Proposed R3 (@ L18) would deal 140 + 75% AD (222 from BC+SG+TH+base AD) = 362.4 damage.
So if instead you proposed that R should scale off total attack damage rather than bonus, then you would nerf the base damage from 100/200/300, down to 40/90/140, and SG wouldn't be a DPS loss on Darius's R anymore, but everything else would remain the same with a standard build.
Of course as mentioned, that would mean a Full Tank Darius build would deal less R damage, and a high AD build would deal more R damage - but I think that makes him better designed and more gear-flexible.