What's a decent number for bike erg cals?

I'm in the midst of coming back from a low back issue so after I finish my lifting, etc, I've just been jumping on the bike erg for 15-20 minutes for cardio. Not going max, but keeping a steady, hard pace. Today, I did 307 cals in 20 minutes. Is that good, bad, decent? I'm clueless on single modality endurance type work.