Doing the sums, what's a good way to model difficulty?

[Reikoku]

I'm currently doing the numbers for how many Bitcoins I can crunch, and struggling to work out expected increases in difficulty, and how long they're going to last.

Can anyone tell me how they're modelling difficulty?

Right now I'm using one change every two weeks, by 20% each change.

[clonedone]

i do every week by a 90 percent increase

[Sukrim]

+40% every 10 days is what I use for rough estimates...

I need to do a more proper model though I think.

[MiningBuddy]

+60% ever 10 days has been a rough figure to work by for a long time.