FTTC line length / mbit formula

Boredom and a degree of nerdiness compelled me! X is your line length in 100m chunks:

Est speed = 115 * X^-0.66


Even less based on fact, for 30a:

Est speed = 230 * X^0.93



Which would suggest 30a would provide major speed boosts for maybe a third of lines (eg a 200m line from 73 to 121), and a decent bump for another quarter (eg 400m 46>63).

The unknowns: how the psd mask would hit 30a, and how the extra 30a bandwidth would be split between downstream and upstream (I guessed similar to present, vast majority downstream).


Disclaimer: large chunks of this were based on wild-ass guesses. It won't work at all below 100m, and above 1000m standard adsl2 speeds should be used.

For those of us who did their gcse maths over fifteen years ago and faintly know what 'to the power of' means, can you talk us through the equation with some examples?