I chose 80%, but to me, new grips or painted sights aren't "modifications", by setting the bar so low you're calling every gun that somebody swapped the grips on modified. That's like saying your car is a hot rod because you put wider tires on it.
I consider a modified gun one that has had action or reliability work at the minimum, fitting and adjustments that actually affect the performance of the weapon over stock. And, IMO, a customized gun is one that has been modified to a much greater degree - internal parts changed, metal work done, refinished, etc - so that the gun is physically different from stock. Of course, the sky's the limit when it comes to customizing.
I have guns that are modified and guns that are full on customs, what I don't have many of is factory stock guns. I prefer mine to be set up a certain way, especially handguns, so almost everything I buy ends up at the gunsmith for a bit of a work over.