Steep Driveway

Simple math, isn't it? How far from the back axle to the low point on the back end? How high is the low point on level ground? The good news is there are rollers and mounts like these thesethat should solve your problem. There are lots of rollers like this on the market... these are just an example. Welcome to the Forum!

If he comes in on some sort of angle it will be less then if backing dead straight in.

Never encountered the need to do this, but reading your description, I'd start with this. Drawing the picture I have in my mind would make it easier, let me try describing ...

(1) Measure the vertical rise at the rear wheel of proposed trailer, until you intersect the horizontal plane where the bumper or lowest portion of your trailer would potentially hit the driveway.

(2) Measure the horizontal distance between your vertical line at the wheel and point of contact.

(3) You now have a right angle. At the start of the driveway, measure the vertical distance, and horizontal distance to see if you hit the pavement before you get your full distance measured.

Follow my thinking? Let's see if others have done this, how close I am to something that works.

I would lay some 2x10 planks at the lowest point at curb along curb to help the first try.