The sweet spot for gaining (where successes and gains balance out) is typically between 60% & 80% success for items. Below that, you lose too much resources per item - above it, you get too few gains. Plus, IIRC, most carpentry items start at 50% success when you learn to make them.
Consider the following - most of the items one makes has little or no resale value.
Therefore, it is best to make items that have quest or librarian donation value, to get more value from the training in the long run. I'd just done a study to this effect on the old forums, listing suggested items to make.
Here's one variation on this - (requires having some music & possibly tailor skill, at least temporarily)
1. Do foot stools to 36, for Heartwood quests
2. Make medium crates until you get up to the level to make ballot boxes (throw away crates)
3. Make Ballot Boxes until you can make Lap harps (throw away the ballot box deeds)
4. Make the Lap harps for the Heartwood quest "Stop harping on me" until you get to a level for other items, for the library(can stick with the harp quest for a while to get recipes while training).
5. Move to library around 70 skill - either make bokutos for the samurai, or fishing poles for the Fisher, there.
6. Sometime at 80+, switch to shepherd's Crooks, donate these to Animal Trainer in the library. Do these until GM.
If you still need recipes after MP, go back to the foot stools.
This method will use a bit more materials than the usual methods, but will only waste items completely in the range between the stools & lap harps. If you feel like getting something "substantial" for your training effort, this is the way to go.