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.