Making a Loop to re feed the Defintion?

Hello guys, I was currently working with a definition that creates a set of circles thorough the edges of a box. I need the circle diameter to be 20units.

But I cannot control its radius, instead I can control the size of the box.

I could simply use rule of three and put the slider in that value.

This means, my current box is 20units, and that generates a circle of 27.88789 units of diameter.

How much does my box need to be to generate a circle of 20units.

I was wondering if there was a way to input the 20units dia. and make that change the size of the box.

Something like inputting the values of the panel to 20 and that run backwards on the definition, making the needed changes to maintain that 20