Rear brake rotor question

[jabstick420]

Just changed rotors and pads for the fronts.

My REAR rotors measure approx. 20mm. vented.

I believe min spec on these is 18.4 correct?

Wanting to just slap on a set of matching pads in the rear since I did the fronts already.

Would this be prudent? or does 20mm warrant new rotors for the rear as well before putting new pads on...

Not sure what these rotors start at, so 20mm doesn't tell me how much wear they have.

I can feel a ledge on the outer circumference but not much.

thx