With no other % armor bonuses (they are additive, so additional % armor bonuses reduces the overall toughness multiplier of Stone Gauntlets) and as your armor approaches infinite, the DR from 250% bonus armor converges on 71.428% (or 5/7). With less than infinite armor, or other % armor bonuses, it’ll be strictly less than 5/7.
Somewhat counterintuitively, the less armor you have, the less effective the % armor bonus is. More base armor means the % armor bonus grants you more bonus armor, which ends up being more effective.
You can convert a % armor bonus to effective DR as follows:
(note that bonus here is the armor bonus modifier, so 3.5 for a 250% bonus, 1.25 for a 25% bonus etc.)
(armor*(bonus)+3500)/(armor+3500) = toughness multiplier for % armor bonus
(armor*(bonus-1))/(armor*bonus+3500) = DR equivalent for % armor bonus
Some examples:
1. 10,000 base armor
(10000*3.5+3500)/(10000+3500) = 2.69x more toughness
(10000*(3.5-1))/(10000*3.5+3500) =64.9% DR
1a. 10,000 base armor with 50% bonus armor from other sources (paragon, skills)
(10000*4+3500)/(10000*1.5+3500) = 2.35x more toughness
(10000*(4-1.5))/(10000*4+3500) =57.5% DR
2. 30,000 base armor
(30000*3.5+3500)/(30000+3500) = 3.24x more toughness
(30000*(3.5-1))/(30000*3.5+3500) =69.1% DR
3. 100,000 base armor
(100000*3.5+3500)/(100000+3500) = 3.42x more toughness
(100000*(3.5-1))/(100000*3.5+3500) =70.7% DR
4. 1,000,000 base armor
(1000000*3.5+3500)/(1000000+3500) = 3.49x more toughness
(1000000*(3.5-1))/(1000000*3.5+3500) =71.4% DR
With infinite armor, this converges to to 3.5x more toughness and (3.5-1)/3.5=5/7=71.428% DR.