180 / 1.4 = 128 (almost exactly)
There's no point in using a lower difficulty with p2pool. You're just wasting your own CPU cycles.
That's about 71% of 180, not 30% of 180.
If you want to get technical, the best difficulty is 32,768 regardless of your local hashrate, because unless about a third of the users dropped out of the network, the difficulty per P2Pool share won't drop that low. Every share found below the current P2Pool difficulty is useful only for local statistics. Unless you're implementing a sub-pool that has a different share tracking method, those shares are wasted.
And every hash you produce before you find a share is wasted? This is the basis of "proof of work", isn't it?
With P2Pool, yes, each share below the P2Pool difficulty is wasted.
P2Pool's definition of proof of work is when you create a P2Pool blockchain block. While using P2Pool, you can create a block on the Bitcoin blockchain without creating a block on the P2Pool blockchain (well, the P2Pool block would be created, but it would be orphaned).
Unless you were part of a sub-pool that used a P2Pool node for its main income, then redistributed the income out to the contributors, then the shares below P2Pool's difficulty are only useful for statistical purposes. This is NOT the same as putting your Bitcoin address as the username when connecting to a P2Pool node.