Check box to over ride formula in another cell

Hi, and welcome to the forum

What's stopping you entering a number directly? Why do you need a checkbox? Why indeed do you need VBA?

If you want to retain the formula but be able to vary it then why not change the formula e.g. D22 to say

Formula:

=IF(A22="",Equipment!C25,A22)

Then if you want to override the formula enter a number in A22.