Salvete Sodales!
I think I found an error with the amulet of longevity in Hedge Magic.
By the guidelines a bonus of +2 would have a basic level of 2. +3 for duration season makes for a level 5 effect, not 10. Alternatively this could mean a bonus of +3. Or did I miss anything?
Vale,
Alexios ex Miscellanea
You are right. There are a couple of problems with the entry. The Level is wrong. It should be a 5 effect and the number of charges should read 0. But because it's technically a device and not a spell, the Lab Total should be listed. You would needed a Lab Total of 21 to create it with the necessary charges. (21 LT - 5 effect) / 5 = 4 charges.
Likewise on that same page (HMRE, p. 94), the Amulet for An Ample Harvest is wrong. It list 2 charges, but a Lab Total of 20 will only get you one charge. You need a lab total of 26 to create the device as described. (26 LT - 20 effect) / 5 = 2 charges.