Position sizing advice?

Ok, I am in the testing phase of my new trading plan. About 75-80% of the time my trades hit the initial stop-loss. I’m cool with that though because I am at a 35-40% ROI. The problem is I am not pyramiding my position sizes on good trades and I feel like I need to. Anyone have a good, low risk strategy to add positions to winning trades? I trade with a modified version of the Turtle Trading Plan using signals for breakouts. Any ideas for adding positions???