Resizes all open windows to a specific width, the last window (furthest right) will be left with all left over screen width
function! WinOneWidth(width) abort
exe "normal 99\<c-w>l"
let l:last_window = winnr()
exe "normal 99\<c-w>h"
while winnr() != l:last_window
exe "vert res" a:width
exe "normal \<c-w>l"
endwhile
endfunction
"Example:
nnoremap <leader>+ :call WinOneWidth(80)<CR>
nnoremap <leader>= :call WinOneWidth(90)<CR>