I hate to say it, but your 9% is correct, for the maths you've supplied at least. You're creating an overall % using that formula, not an average of your results (sorry I'm a maths lesson here):

Your maths:

[font="Courier New"](1.5 + 0) / ((11-0) + (11- 5.5)) = 1.5 / (11 + 5.5) = 1.5 / 16.5 = 0.09 = 9%[/font]

What you're going to be looking for is:

[font="Courier New"]((1.5 / (11-0)) + (0 / (11-5.5))) / 2 = ((1.5 / 11) + (0/5.5)) / 2 = (0.136 + 0) / 2 = 0.068 = 6.8% ˜ 7%[/font]

This is a little harder without your code, but I'm assuming you have a cell in SSRS with something like (I have named your three columns [Base], [Finish], [Start]:

=Fields!Base.Value / (Fields!Finish.Value + Fields!Start.Value)

You could, therefore, simply put an AVG() around this formula in your expression in your totals grouping. This will give you the 7% you're looking for.