Here is the formula set up to solve for MGD (Million Gallons per Day).
=3*936*A21^1.5*1440/1000000
[....]
What I want to do is set up another formula where the desired MGD
could be entered in a cell and the result would be the inches to let
flow over the weir.

[...] is this simple algebra? If, say, B21 is the MGD and you want A21
to be the inches going over the weir, then:

A21^1.5 = B21*1000000/3/936/1440

I certainly prefer Rick's approach of taking 2/3 power of both sides.
Not sure why he has two variables, where I have one. Perhaps Rick has
given this more thought than I have. But I would say:

I thought you might like to use this formula in the future for other
weirs... the odds that the length of every weir you will deal with in the
future will be 936 is pretty slim<g, so I used a variable for that term.
The remainder of your simplifications is fine... I didn't simplify the
constants because you didn't in your original posting.

Rick