Optimisation of a variable

I have a Mathcad document along the lines of:

r:=10mm

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Many equations using r and other variables.

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stress = 50 MPa

Is there a way to find the value of r that gives the minimum stress without totally re-writing the document?

I know this can be done relatively easily for discrete values of r by using a vector but I'd ideally like to do the equivalent of an Excel goal seek.