You probably saw from my WIP tutorial I did about me having the same issues. Even though I used a different steel, I got a similar situation that you are in. I knew 40 was probably to low and would most likely spring like Chuck said is possible..So I re heat treated. I think I ended up with about 45 or 48..can't remember what it was now without looking. The thing is, too hard and it breaks. Too soft and it springs. The hardness is directly related to the pressure required to open and close the blade (the amount of tension you feel) but also remember this can be altered by the amount of material taken of the tip of the spring where it contacts the tang area and pivot on the blade.. Alot to think about..
Like I've said before, it would probably take years of mking these things and 100's of knives before you could pull one off without issue. There are just so many areas for problems. Good luck. I can't wait to see your completed knife.