You, my friend, is on the path that will brings you only pain and sorrow. Either you have to break the game code or you will have to prepare tons of energy, time and effords to even come close to something that Is reliable.

There are people out there that tried that and the best efford I am aware off is no near complete and is abandoned for an year from now.

The formula itself is not that difficult to be "extracted" , the actual numbers are the missing part of the puzzel. 

ive managed to calc the total stats, is it that hard to get the power from there?

how important is it anyway?

I do not think that power is a good indicator of how powerful a champion is. Power scales a lot with critical rate/damage and resistance. But it doesn't scale up that much with speed (arguably the most important stat in the game). Also, we have to consider that HP, Attack, and Defense scale different amounts of power depending on the champion that you have, whether is HP, Attack, Defense or Support based. The best thing, if you really want to test it out, is to farm for flat stat pieces on the campaign, do not level them up and see how much every flat stat changes a champion. 

Say: 25 Defense will give my character 250 power, because he is Defense Based, and 25 Attack will give him 75 power. Do this for every single stat and perhaps assume that the increment of power is linear.

We do know that the percent of damage reduction after 3k defense, starts to dramatically decrease. It is exponential at first, then it becomes linear for a bit and then it becomes a function that is similar to y = square_root(x) being y: damage reduction % and x: the variable that is defense. So, considering that power DOES NOT follow the same principle then you can use a rule of proportionality for every single flat stat.

Hope this helps you!

Wait, you are interested in power rating, not the damage scaling, right? Then ignore my previous post, I misread your post and my answer doesn't make sense. Sorry about that. 

You can't get exact numbers, 'cause in this game nothing is public, unless the drop rate on shards.

But the power depends on resistance, crit rate/dmg and main stat (the one indicated in the skill description).