Curiosity is the mother of Invention. That makes Boredom it's grandma.
I've ebarked upon discovering the rules and formulae behind the crafting level XP progression.
As a source of data I've used the data available on DDO wiki, copied from there at around 09:00 GMT.
Below are the results I've come up.
Step 1: Plunging into derivative waters
Strictly speaking we can not make a derivative of a discrete function, but we can do something very similar: we can look at the differences between the neighbouring points.
Using the formula (XP(level+1) - XP(level))/((level+1) - level) = XP(level+1) - XP(level)
I got the following result:
Here, with a careful look, we can already make some interesting observations.
It seems the XP progression is not a smooth function of the crafting level, but rather a composite of a few different ones.
-Levels 1-10: Function looks very linear, which would indicate an arithmetic progression of xp/crafting level for these levels.
-Levels 11-36: The function here is clearly non linear. This might complicate deducting a formula in the level range and lead to some guesswork.
-Levels 37-42: There seems to be an anomaly here, which could just be the result of incorrect data in the DDO wiki's XP tables.
-Levels 43-60: This again looks linear, but with a different slope than at levels 1-10.
To make sure of my findings so far, I went further.
Step 2: Reaching the derivative bottom
Using the same formula as in the previous step, we can make a sort of second derivative of the initial XP@level function.
The results are:
While the level ranges, that are using the same formula, could only be guessed in the previous graph, they are clearly visible in the second one.
Further conclusions can be drawn:
-Levels 1-10: Function governing XP progression in this level range is indeed linear making the initial XP@level requirement a sum of an arithmetic progression. In level range 1 through 10, the XP function is a parabola.
-Levels 11-36: The second derivative is linear. In turn, this means that the initial XP@level function will be a 3rd degree polynomal in this level range.
-Levels 37-42: The anomaly is now even more extreme and easier to spot.
-Levels 43-60: Similar to level range 1-10, the initial XP@level function in this range is going to be a parabola (but a different one).
The only thing left to do now is to determine the formulae parameters.
Step 3: Fishing outt he formulae
Since there is a different formula governing the XP progression for every different level range, this will have to be done for every level range individually.
Levels 1-10:
We have the value of the second derivative in this range, which is a constant.
It alternates between 3 and 2, so I've taken 2.5 as the correct value.
Integrating the the constant twice (which I won't do here, because lacking the appropriate symbols, it would only be confusing and a pain to do) and applying the boundary conditions to determine the free constants, the formula emerges:
XP = 5/4*Level^2 + 11*Level
Boundary conditions used are of course the given XP@level values and the calculated differences.
Levels 11-36:
This one was just slightly more difficult, since some corrections had to be made to the factors after each integration, because of some approximations in the previous step.
Nevertheless, using the same procedure, I've determined the formula governing progression at levels 11 to 36 to be:
XP = 1/12*Level^3 + 5/8*Level^2 + 2/5*Level + 85
Levels 43-60:
This one was done in completely the same manner as the formula for levels 1 to 10. The result is:
XP = 5*Level^2 + 5*Level - 1862
And people say that math is not fun! Pha!