Hi @create , you can achieve this by applying a grid to your main section with 1 row and then in the sizing options you can use the calc function to set the it to:
calc(100vh - theSizeofYourFotter in pixels) like this -
then your main section will always be 100vh minus the size of your footer.
also you need to take in consideration that if your site is not premium at the moment you also have to calculate the extra 50px of the editor x banner at the top.
Hope this helps.