All on-hit ADCs like Runaan's, so it should be about 60% crit chance.
And, either way, the idea isn't to keep stacking crit items if you pick Rageblade. It's the opposite: you are supposed to build it when your champion is going to sit on low crit chance, so you convert that somewhat 'undesirable' stat into something consistent and of value.
Rageblade's design is fine, the problem is on the numbers. They nerfed both the cost and the damage output from it, 'cause on-hit ADCs were outperforming their crit peers in early pre-season.
Now that IE got adjusted to be a better 3rd item and other crit items got some buffs, they should roll back the nerfs they gave to Rageblade.