You may be able to alleviate the timing issue by adding as you go. On the very first page, set your memo_end to be 0.
I assume you have some method for the user to navigate from page to page, like a Continue or Next button. If so, then you can assign a new action to the button to add memo_x to memo_end as the person activates the control that navigates to the next page. That should eliminate your problem of the user clicking on different choices and modifying the final score.
By the time you get to the end, you will need to add just that one final variable to memo_end. That should give you near-instantaneous results.