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I used 1.5 as the adjustment factor, based on the fact that my 5" speakers would be 1 and my 12" speaker would be 2, and this was halfway between. The only way to get more accurate, would be to measure actual specific drivers between the outer edge of the frame and the center of the surround, then double that number and use it in the calculations. Alternately simply measure from the center of the surround to the center of the surround to get the true piston diameter of the driver.
I also performed the calculations in an EXCEL spreadsheet, and verified the first calculations using a calculator to make sure I had entered the formula correctly in the spreadsheet.
Here are the results. This is a closer approximation of the true piston size of a range of drivers 
4.00" = 4.91 sq.in. = 1.00
5.00" = 9.62 sq.in. = 1.96
5.25" = 11.05 sq.in. = 2.25
6.00" = 15.90 sq.in. = 3.24
6.50" = 19.64 sq.in. = 4.00
7.00" = 23.76 sq.in. = 4.48
8.00" = 33.18 sq.in. = 6.76
10.0" = 56.75 sq.in. = 11.56
12.0" = 86.59 sq.in. = 17.64
15.0" = 143.14 sq.in. = 29.16
If we compare a 6.5" and a 5" using this new chart, we have 4/1.96 = 2.04. So, a 6.5" is 2.04 times bigger than a 5" driver.
If we use the previous chart that simply used the Rated Dimension and made no adjustment, then compare the two same two drivers, we have 2.64/1.56 = 1.69 or 1.7. Implying that a 6.5" is is 1.7 times bigger than a 5".
If we compare 3x6.5" against 2x8" we now have 
three 6.5" 
3 x 4 = 12
two 8" 
2 x 6.76 = 13.53
13.53 / 12 = 1.1275 = 1.13
So, using these new numbers, TWO 8" drivers are about 13% larger than THREE 6.5" drivers.
Using the original Rated Size unadjusted chart we have 8 / 7.92 = 1.01, or the 2x8" is 1% larger.
I think either way, you can get a rough idea of the relative difference between two different driver configurations.
Which you use is up to you.
Steve/bluewizard
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