New to middling - how big of an edge is needed

[Recliner]

Sharp Sports Betting has a table that shows the "Frequencies of Pushes Against the Spread" that is helpful determining the value of middles. (Page 234, for those playing at home) FYI 3 is listed as 10%. This is also helpful when determining when to buy points and calculating how much they should be worth. I would also think that you should be able to find a similar list somewhere on the internet.