VBA recalculates formulas even when stepping through unrelated code?

I have a program that uses the following line:

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When I step through the entire program and watch this:

WorksheetFunction.NormInv(Rnd(), Mu / 100, StDev / 100)))

I notice that it generates a new random number and recalculates NormInv on each step. I may be wrong but I believe the calculation of NormInv is done through an iterative process which I am guessing eats up a lot of clock cycles. The only time that I need the program to generate a new random number and calculate NormInv is when the program gets to that line. How can I get NormInv to NOT calculate a new number every step through the program?

I tried to modify my code by creating a function that uses rnd() to store a random number in a variable but I must be missing something because as rnd() changes, the value of my variable changes with it. How do I get the variable to update its value only when the function is called?

Here is the code:

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