Target a window for the difference of MTP over 6 blocks - the same
quantity used in the current EDA.
The window is chosen so that with stochastic randomness of block
times the block interval will average approximately 600s.
The bump sizes are chosen to:-
a) make a difficulty ramp attack from temporary excess hash power
a lot more expensive to attempt than to recover from
b) minimise the maximum time between 2 blocks over long-run simulations
of hash rate coming and going, including in large chunks, and randomised exchange rate moves