chart with two y-axis

i am trying to design a chart with 3 axis (2 yaxis and a xaxis), where if you know the values on two of the axis you can get the values of the third.

i have attached my spreadsheet as well as a picture showing what i would like chart to look like.

for example, (see attached pic) i would like to be able to do this... i know Q=0.400, So= 0.100, and DEPTH= 0.100
and i want to be able to read off the other axis that WIDTH = 2.000m

i am having trouble trying getting the two y axis to correspond values.

sorry for the confusing description and thanks for help in advance

DESIGN CHART.JPEG

gutter design chart.xlsx

I assume at some point the relationship must get more complex as current Width is x2 the depth.

Anway you can calculate the values using some simple maths and INDEX and MATCH functions to get the known values nearest to the inputted values.