My Little Red Rain Gauge is Tilted

I noticed that the rain had collected in my little red wagon, which also serves as my rain gauge in the back yard. Because the wagon was tilted, a shape formed like the one drawn in the picture. Accurate measurements were taken to reproduce the shape in scaled manner and the shade of blue adjusted to show depth.

There were a couple of right angles involved that could accurately integrate the volume of water without the use of calculus because the triangle shapes changed in a linear manner going from one side of the wagon to the other.Then it would just be a matter of dividing the water volume back into the total surface area of the bottom of the wagon to calculate the amount of rainfall.

Linear measurements in centimeters

3.1     48       74.4   area of smaller rectangle 
4.3     74.5    160.2   area of larger rectangle 
117.3   ave area of a triangle 
35.3   Width of wagon 
Volume = Ave Area Triangle x Width of the Wagon
4140.2   volume of water cc 
35.3    82     2894.6   sq cm area of bottom of wagon
1.430 cm, height of water distributed evenly 
0.56  in, height of water distributed evenly

It rained about a half inch.

Additional Work

I played around with the math a little more using numerical integration and I think the average area of the triangle is close to 114.64 sq cm using 40 regions of numerical integration, so the final answer may be 2.3% lower.

It still rained about half an inch.

Additional Additional Work

In a perfect world, a perfect wagon, & perfect measurements, I believe the triangles would be similar, so there probably wouldn't be the need for the numerical integration.

Yeah, pretty sure it rained about a half inch.