Jonathan, what about this calculation?
Assume Cash is at 50% of Portfolio Value, and the minimum buy is 5% of Share Value. Assume further that you use 10% Safe both sides. If you have one buy and sale a year, 5% of your cash has made about 35% in that year. So you can safely add .7% to your cash returns for each round trip/year in this situation.
Now assume Cash is at 25% of Portfolio Value, and the minimum buy is still 5% of Share Value. If you now have one buy and sale a year, 15% of your cash has made about 35% in that year. So you can safely add 2% to your cash returns for each round trip/year in this case.
Regards,
Karel