I don't have a 12.5" but I have 11.5" and 11.75" rifles and I also run a 10" rail on my 11.5" rifle.
I use a 1-5× Leupold Firedot on one and a 1-6× PA NOVA on the other.
My 11.5" is a 300 BLK so it only gets shot out to 300 yards.
You can open up plenty of railspace by mounting the latest generation MBUS front sight at the rear of your rail, under the overhang of the LPVO objective. The use of a short rear BUIS allows the whole optic mount to move back and limit your rail space interference at the back of the handguard.
As a guy with longer arms, larger hands, and run the fore end with a thumb over bore grip with thumb pointed straight down the muzzle, I feel I have all the room in the world with the 10" rail if not running a laser for night vision. I use a Modlite in an Arisaka offset mount on my weak side, tailcap activated, no pressure switch. When mount a laser I am going to have to go with a MAWL to keep the same grip or mount the laser on the strong side and run a tape switch at the front of the rail. The rail space is such that a LPVO with overhang would necessitate that or switching to a VFG, which I do not prefer. On the up side, the MAWL will stay out of my FOV with a LPVO, where a longer rail and traditional laser with either a switch behind the laser (if light mounted weak side) or laser switch behind the laser (if I keep the tailcap activated light strong side) would block a bunch of my scopes FOV and might even prompt a higher than 1.5" scope mount.