I don't have any evidence, but I always thought that each Tailed Beast was exponentially stronger than the previous. Something like 10x 1-tails = 1x 2-tail.
If anyone has evidence of this, edit it in the answer.
According to the questipon provided by JNat
The equation could be like this: (x)t = beast w/ # of tails
(Addition)
1t + 1t = 2t
1t+ 2(1t) = 3t
1t + 3(1t) + = 4t
...
1t + 8(1t) = 8t
1t + 9(1t) = 9t
9t/2 < 8t
In which case the 1/2 of Nine Tails wouldn't be stronger than the 8 tails.
If it were like this, then the equation could be more like:
8! (8*7*6*5*4*3*2*1) = 8 tails
9! (9*8*7*6*5*4*3*2*1) = 9 tails
9t/2 > 8t
or it can be
8t = 7t + 6t + 5t ...
9t = 8t^x
9t/2 > 8t
x being the number of times stronger in which each tailed beast is stronger than the other.
This all depends on whether the tailed beasts are exponentially stronger than another or additionally strong