Calculate width from bottom mobile menu to top for SIDE menu

Hi gents,

I need your help, searched for forum but couldn’t find solution thus far. I’m building my app for almost 1 year now and it’s ready soon to go out, but now I want to optimize performance. So far I managed to find everything on forum, but this is my first post as I could not deal with it.

I am using the right menu plugin for mobile users, but I would like to not do it with plugin, as it’s slower and can not be modified the way I want, but rather with group focus or some other way.
Let me explain:

RED marks bottom floating group, that is menu. When I click on last item on the menu (bell icon atm), I would like to open another menu that is marked with GREEN color.

I can make this happen when I put Group Focus on click of the icon, let’s say -500 pxs, and then it will show the menu, but it will never be 100% height of the screen view.

How can I calculate this number of pixels so it always fills up entire page.

(Don’t mind the yellow marking).

Thank you very much for your help gents.
Cheers

Any help here gents? Still struggling…