Change your difficulty - nearest power of 2 of 30% of your hashrate
(so +8 for a LS, +16 for a Single)
Where is this set?
There'a lot of really useful information in the p2pool documentation ...
It doesn't seem to be an option in p2pool or bitcoin
In your mining software by appending +(n) onto the worker name (address)
And what is the formula?
The one you quoted

A Single is 60GH/s so 30% of that is 9
30% of 60 is not 9 for a start - and the numbers for a single and a little-single :p
30% of 60 is 18, nearest 'power of 2' to 18 is 16, so you use 16