Variables - allowing only one click to add a number

Hi guys,   I have added a progress scale to a session.

This uses the information as per your tutorials - which are great by the way.

In my scecario, there are 5 branches to five different sections and when the click a 'complete' button at the end of each branch then the variable adds 1 to the score and 20% of the scale is shown through a state change. This all works fine.

The problem is when you view the whole session, and can freely take 'prev' buttons, review a branch  - and then, of course, another 1 is added to the vasriable - so you can get a false progress.

How can I limit a count to 1 for each 'complee' button so people browsing back through previously seen slides does not affect the scale?