Slope and Area Calculation help

[FullSpool]

I am trying to calculate an area of ground that has a 2.5 to 1 slope. The slope changes in length across a mass area. My question is looking at it from a top view I can see where the lengths of the slope change, I can flood that area and use CAD tools to get the area of that region. However that isn't a true representation of the slopes actual area of the total length as it is a top view and not looking at it from a section view. Is there a way of getting a multiplier to get an approximate value that will get me close instead of calculating a slope length in every section view in 1" increments?
Since my slope stays constant at 2.5:1 can I just use c2=a2+b2 to find the hypotenuse then divide 2.5 into c2 to get my multiplier ?
So if my hypotenuse is 2.69" I can divide that into my slope giving me 2.69"/2.5"=1.076 as my multiplier
Now I can take my area 68.1 sq in. X 1.076 to give me a an approximate area for that distance as if I was calculating it from a section view?
Will this work or am I way out there? It doesn't need to be 100% accurate just within 10% or so for now. I will do the section calculations when it's more relevant.
Thanks for any input!

 

[CarlB]

Your approach looks ok. I get a multiplier of 1.077 for a 2.5:1 slope, used to get slope length from horizontal length.
This shows that you would be within 10% even without accounting for slope, but you should be as accurate as practicable, then add an estimating factor at the end.

 

[chicopee]

You could divide the surface area into equal width and add up all the rectangular areas developed on that surface.

 

[racookpe1978]

Are you worried about total volume of the enclosed "dirt"? Or exposed area of the top of the "dirt" to atmosphere?

You have an undefined (irregular) edge that will be more important than the slight slope.