Newb question re halftime middles

[dmck0027]

Hope I don't get flamed but I couldn't find anything about this on the search...

I was wondering how to determine whether to middle with a 2nd half line on a bet that looks to be on track to winning easily.

An example: tonight I had the Celtics -2.5 against the Heat and the Celtics were up by 20 at halftime. I wasn't by my computer, but for the sake of this example let's assume Miami was -3.5 in the 2nd half. I can bet Miami in the 2nd half to set up a 14 point middle. Considering that Boston seems to have the 2.5 game spread under control, should I always make the Miami 2nd half bet, never, or is there some other methodology that I should use to determine if this is a good bet?

I apologize ahead of time if this is a retarded question. Thanks.