Measuring difficulty when you can't hold an erection long
That’s a problem a lot of guys doing penis work report. You can get a full erection, but by the time you fumble around with the ruler it’s half gone. You’re not alone if that’s happening to you. Here’s a solution.
Get a masturbation sleeve, a ruler and a block of stiff foam. Slice a slit in the middle of the block of foam with a knife and slide it over the ruler turning the ruler into a depth gauge. Ideally you want it tight enough that it won’t slip up and down on the ruler under it’s own weight. You have to move it with your hand. Just above the end of the ruler, duct tape the sleeve to it so the tip of the ruler sticks past the opening a bit, so you can bone press the ruler without bottoming out the sleeve. Cut the sleeve shorter if the tip of your penis doesn’t stick out the end.
Stroke yourself until you are the hardest you can get, bone press the ruler, then slide the foam block down until it touches the tip of your penis. Take the whole thing off and read the measurement. You’ll always be measuring at your absolute hardest that way.