Seeking help calculating the value of a half point in MLB R/H/E Prop

If anyone could please help me calculate the value of a half point in the MLB Runs/Hits/Errors proposition bet that is commonly offered, I would be eternally grateful. For a real life example, if I deem the "correct" line to be 26 R/H/E for a certain game, and my local is offering, say, 27 under -120 on that prop, should I play the under or is the 20 cent vig too much to still have a positive EV? If I knew the approx value of a half point on this prop, I could answer the questions, but I don't know how to calculate it.

Any help would be greatly appreciated.

To the Wiz: Cool forum!

I would guess you don't based on the fact you are asking this question, but it can't for me to ask: do you have any data for the R+H+E of a bunch of games (even better if it also includes the R+H+E line before the game)? I can maybe help you but I'm not particularly interested in finding the data myself.