Calculate Percentage of task completion with Checkboxes

I have a project management system I'm working on and need to calculate percentage of task completion based on tasks checked off. I've tried using the formula:

The formula below works, but the problem I run into is not all milestones have five tasks. I need a way to factor in the variable of blank cells. I've done tons of research but nothing seems to apply to my specific situation. This site is great and I really appreciate those of you helping those of us who are technically deficient.

=COUNTA(B5:B9)/5

The cells reflect the following:
A B
Task 1 check box
Task 2 check box
Task 3 check box
Task 4 check box
Task 5 check box
TSK CPLT PERCENT : %

That formula works partially. If all the boxes next to a task are checked the total is accurate. However, if only some of the boxes are checked the formula returns a faulty value. For instance, I have one milestone that only has two tasks of the five... with both boxes checked it returns a value of 100%.... with only one box checked the value is %200.

Sorry to be a pain, but I'm stoked that I'm almost there... I'm sure one of you smart folks can help me solve this. Thanks so much for the quick reply as well... I've been up all night trying to get this thing right.