Buying Points and Related Issues/Sports Betting Theory Question

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"Hypothetically", Team A is favored against Team B by 5. I think Team A has a good chance of beating Team B by 20 points. Therefore, I am looking to get action on Team A at a point spread higher than 5. Suppose I could bet on Team A, give 5 points at -110. How do I go about determining at what number I should get to increase the line to 7? Related, if I can get the line at -10.5, how do I figure what fair value is?

I'm looking here not for some sort of general "feeling" type answer, but more of a mathematical approach.

Syg? Anyone?