Peter, I typically use no more than 30:1. Generally, It's about 20:1.
I use Oanda... not bad for trading tens of thousands... not sure how they will be at hundreds...but I'll see.
If need be, I'll open an account with Dukascopy, or some other top level bank with a direct line to the interbank market... but for now, the bucket shop that oanda is works fine for me.
Almost never fill problems, very little (no?) slippage, spreads decent, except around news times when they are horrible... so I just exit 30 min prior to news, and don't get in until 30 min after.