Summary (aka. things I learned from Monkey and Mathy)
1. there is no straight-forward algebraic method...books consider the historically-based probabilities of similarly-lined games winning/losing/pushing and adjust the line according to the changed probabilities (and their need to maintain the same vig "markup")...
2. best thing to do when u need to know if an alternate line is ok...is to refer to books with a generous menu of different-numbered lines (Pinnacle with the sharpest lines...Bet365 with the widest selection if you need to consider something further away than Pinnacle makes available) and use the %differences that apply to the number-changes you're interested in...
3. the half-point calculators available here and elsewhere are a guide, but if not periodically-maintained (they very likely are not) to include recent results (that books definitely incorporate into their line-setting protocols)...they grow less and less dependable as such
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I hereby declare that this thread has fulfilled it's purpose...