I gave you proportions, not hard and fast measurements. If your coop is up 4', then the horizontal distance would be 4 x 4/3 or 5-4". The length of the ramp would be 5 x 4/3 or 6'-8". It does require you to do some math with proportions. I could have been clearer. If you tell me what height the coop floor will be I can give you the measurements.
You do not need to use sandpaper. By cleats I mean something like 1" x 1" sticks of wood fastened across the ramp for their toes to grasp.
I don't know what breeds you will have. Some breeds like Silkies can't fly or some have other issues. Mine not only can hop quite well they have no problems flying up to 3' or 4' high nests or to roosts 5 feet high and back down. Mine are full sized fowl, not bantams, and can pretty much step up a foot without hopping. You can position the top of your ramp a little below the bottom of you pop door. You can put the bottom of your ramp up on a cinder block.
People really like their ramps and can be particular about them. Sometimes they are necessary.
@aart has some good photos of the ramps you are looking for. I'll show you a photo of mine. This is to my grow-out coop, which is maybe 3' off the ground. I put chicks as young as 5 weeks in it. If they want to they can fly up to and down from the opening and many do. But some do like using this ramp. Let's just say I'm more laid back about a lot of this stuff than many others.
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In this one I was using a 2" x 2" instead of a tree limb, but this shows the entrance to my grow-out coop. I don't know if it puts it into perspective or not.
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