With all due respect, the only reality is that your logic and methodology are faulty. I will give a hypothetical example for a 2 team parlay (to save time and space). As in original post, only 2 choices, the favourite or underdog allowed.
Game 1 Miami +7.5 at New England -7.5...
The following method might help you with your model. Using the lines posted by the book, you can remove the juice and figure basic probability of each team winning the game.
Say have a game Boston +120
implied probability for...