Calculating Half time middle probabilities?

[PropPlayer]

Does anyone know of a site that offers data or a database service on NFL half times. I’ve been finding quite a few middles but am not sure how to quantify the value.
For example I right now I’m risking $37 to win $650 that the Rams/Cardinals second half scores will be exactly 21 pts. Good bet? Bad Bet? I have no, does anyone know how I can go about finding out. In this case I'm fairly sure its a good bet because I'm clearing a bonus, but would love to know the value for the future.

[sfwusc]

I dont know the exact odds, but I had 7 being the margin of victory around 10% and hit the half middle on it today. That was a nice score, and special thanks to JAX for score that TD with 3 minutes and change left.

You might can find the sharp betting book that was post ealier this week. It is very helpful.

-SFWUSC