I always thought that was it - air temperature, but someone told me it was 5 degrees (soil temperature) and I think that makes more sense, air temperature can vary so much, night to day etc. The agricultural colleges might give you the answer.
I think if I had a pony prone to lammi I would be keeping him off grass permanently - maybe let him out when the grass has gone and other horses are needing to be fed hay. I did hear that frosted grass is worse - it continues to photosynthesis and make sugars but doesn't grow so that the sugars are more concentrated, the only real answer is a bare paddock and hay if he is that much at risk
It's a bit tricky as his was a stress laminitis after an episode of lymphangitis EMS and cushings tests negative. I don't have the option of a completely bare paddock. I will have to strip graze the paddock down. If off grass completely he only has access to a small yard. I use that by day currently and will swap to over night at some point.