I'd like to suggest a different approach to this type of problem: (I hope I didn't misunderstand it)
To me it seems like the current approach is: "lets try to make something and see what we end up with and if it is acceptable."
In this case: Lets create a formula and see what crafting time it gives us and then we will decide if it is acceptable.
How about turning the question upside down and first decide for a goal (ie. what crafting time is acceptable at a given level). Then you can figure what parameters you can use in your formula and from that finally construct a formula that fulfill that goal.
Actually I think the above question is a part of a long chain of questions needed to be asked. First:
How long should it take to reach a certain level if you do it the most optimal way without interruptions?
I know it is a difficult question to answer because it may be an unpopular decision to make. But I think you kinda already do make that decision by testing what you have made and decide if it is acceptable. It is just a matter of the order in which you make the decision.
You may already have done all this. But then i think you need to give us some more information about the goal to work with before we try to give our opinion about a solution.
So, to sum it up:
- Set a goal.
- Figure what parameters can be used and/or tweaked to achieve that goal.
- Create the solution (here the formula).
- Test and see if the goal is achieved otherwise return to 2
- Profit
- Kia