So for example,

I see a Cho'gath with 100 armor and 30 MR. Well, I know that 100 armor is 50% resist off the top of my head, but I have no idea what 30 MR is (30/130 = 23%) ... if I am playing a nuker and my nukes will do 585 & 200, 30 MR requires more mental math than 23%.

I was recently reading the Veigar thread, it occurred to me that while on casters, I'd really rather see the % MR of my opponent rather than the flat value. I mainly want the option on some chars I play, not all of them!

Thanks!

I'd be up for that. Or if not 'instead,' at least put the %-reduction after the armor value in parentheses. %-reduction is already a number created in the game, viewable if you mouse over your armor value, so I don't imagine it would be very hard just to print the %-reduction after the numeric value.