Formula for a horizontal cylinder

Hello all,

I'm working on a project for work and I'm stuck. We have a 500 gallon cylinder that is on it's side that we store oil in. The manufacturer provided us with a printed table that will give us the volume based on whatever the reading is. What I'm trying to do is create a sheet with the formula so that all we have to do is input the measurement and get the volume without referring to the sheet. I think I found the formula, but I can't get it to work in Excel. Can someone help out? A fresh set of eyes always works wonders.

volume= R squared cos to the power of -1 (R-H/R) - (R-H) square root 2RH-H squared.
R would be the radius, H would be the height.

My tank is 64" long with a radius of 24". According to our chart, a measurement of 40" would mean that we have 446.43 gallons of oil in our 500 gallon tank.

Everytime I get the formula to work, the answer is incorrect. When it doesn't work, I get the error #NAME?

Any help or suggestions would be greatly appreciated.

Awesome.
Thank you so much. That worked perfectly. I had no idea where I was messing up.