Math question for any engineers out here

trcothorn wrote:
I ran a string line to measure my retaining wall height. If I have the line 3 feet above the ground and check the line for level, it is still just barely sloped back. I do not want to build a wall taller than 3 ft due to increase chance of the wall leaning or giving way. So I went inside the trailer with my 4ft bubble level and measured, if I rise the back of the bubble level 9.5 inches it shows level.

So my question is, for every 4ft there is a 9.5 inch drop. My trailer is 30ft. So do I divide 30 by 4, and times that by 9.5 to figure how to get level? Doing that tells me I need a 71 inch wall to get level. Am I doing this all wrong or over thinking it? Really would appreciate input on this my brain is hurting.

Your calcs are correct, however a string run 30+' from ground level at the front and 3' above existing grade at the back was not "barely" sloped back. Your trailer floor is at LESS of a slope than your grade because it is tongue low. Therefore, your actual grade is a bit steeper than what you measured in the trailer, if you measured that correctly.
Pictures are deceiving. Your pic "looks" like a 3-4' tall wall at the back of the wheels will give you a pretty level pad. But start over as I suggested and run a level line from existing grade wherever you want the start of the pad back to where you want the back of the pad and start from there.