Lay systems question

[wb]

Most of the systems I create are win ones. If they are consistently profitable to SP, it is hoped that the profitable trends will continue especially with exchange prices. However, I find it much harder to create lay systems. Normally I add around 15% to the sp to back test it, but this is a bit hit and miss.

What minus ROI (for the backer) do people here look for before deciding it's a good lay system?

[Win2Win]

At least 30%, pref 40%+, this takes into account numerous variables to get over the threshold involved in profitable laying, hence it being MUCH easier finding WIN systems.