Measure progress with a condom
I’ve been thinking of doing this, becoming fully erect with a condom on and marking off the point where the condom reaches the base with a marker pen. Of course it wouldn’t have to be the same condom every time you measure (but the same size/brand), I guess if you can roll it back up afterward then there’s no reason why it couldn’t be. After months of PE you could just unravel them side by side and marvel at your progress.