Calculate temperature of kanthal wire per electric Watt
Basically, it turns out there is no simple way of finding an answer.
However, I was wondering if I could find a function whoose curve passes through, or near, most of the temperature points from the spec sheet table... turns out, yes, there is such a function!
I have actually found 2 functions of interest. The first is a logarithmic function (base 10):

y = 2672 * Log(x) - 2733



This function is the one I found gets nearest to all thermal points of the table, but definitely deviates a lot at the 20W and 27W marks.

The second is a linear function:

y = 61x - 621



This function is simpler, not involving a Log. However, it also deviates the most compared to the Log one, noticeable at the 22W, 27W and 35W marks. This is the one that I ended up using. I could then calculate that:
11.8W = 100°C; 13.5W = 200°C. A quick math gives that my project needs a minimum of 3 Li-Ion cells in series, capable of 1.1A continous discharge current (max needed for 200°C).

I guess that, if the thermal expansion table would've been taken into account, a better estimation would've resulted. For some estimations however, it's not worth the trouble.

Hopefully this will be useful for someone else.
And thanks to anyone here who tried to help me.

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