hydraulic elements graph problem

I'm stuck on solving the problem below:

A 72 inch diameter vitrified sewer pipe (n=0.014) is laid on a slope of 0.00025 and carries wastewater at a flow of 50 ft^3/s. What is the depth of flow?

I think I have to somehow follow the mannings n graph line from the Hydraulics elements graph for Circular Sewers but i'm totally lost here. Can someone provide steps to this solution? thank you!

--If you have the CERM (16th edition), look at page 19-5, section 7, Example 19.2. An example is given on how to solve these problems. It may be done by either the Hydraulic Element Chart or Circular Channel Ratios. It may be necessary to calculate Qfull or Vfull to solve the problem. Qfull = (1/n)(A)[(R)^(2/3)](S^0.5); A = Afull = the area of the 72 inch pipe!!!

@Acedeala Thanks for pointing me in the right direction! That's exactly what I ended up doing! I used mannings equation to solve for Qfull (substituting with the Afull value) and then used the ratio of the given flowrate (Q) to Qfull to get the value from the graph for d/D. 

I also just looked at the example in the CERM that you recommended. I noticed in the problem they mentioned "n varies with depth." In my problem, it doesn't mention if n varies with depth or if n is constant. The hydraulic elements graphs has two different lines for these cases for discharge (Q) and velocity (v). I wonder if on the PE exam they would actually state which case it is; otherwise my answers would be off! 

@Aurora09

When using the Hydraulic Element Chart on the exam use the lines for constant values of constant n. I doubt they will gives problems with varying n values. If so, you will use the lines for varying n values. I hope this helps.