Percentage Completed with Checkboxes....

Alright, I'm almost there, but I need some help. I have a spread sheet of some task milestones and I want to use check boxes to tick off completed tasks and then have a cell at the bottom that calculates the percentage of completed tasks against those not completed.

I used the formula below which works, sort of.. but it only adds 10% for every task checked off. I want to be able to add or delete tasks and have it impact the completed percentage. Can someone help me? Thanks in advance.

=COUNTA(B5:B10)/COLUMNS(B5:B10)/10