So this absolutely can be done. There are several ways to achieve your goal, I will attempt to explain the what I believe to be the simplest way to achieve your purpose.
In the sketch of your feature that you want to control by equation, double click on the controlling dimension. (assumed from screen cap that you were using a hole wizard but other features also follow the same principle)
With the dimension selected clear the value, then type "=". Under the "Manager" tabs activate the feature tree, select the feature that has your driving value (your length).
Re-select the dimension box so the cursor is active, then select your driving value.
So this absolutely can be done. There are several ways to achieve your goal, I will attempt to explain the what I believe to be the simplest way to achieve your purpose.
In the sketch of your feature that you want to control by equation, double click on the controlling dimension. (assumed from screen cap that you were using a hole wizard but other features also follow the same principle)

With the dimension selected clear the value, then type "=". Under the "Manager" tabs activate the feature tree, select the feature that has your driving value (your length).
Re-select the dimension box so the cursor is active, then select your driving value.

Then type equation.
