Someone math smart out there must be able to come up with an easy formula for adding up how many gnomes have been killed given two numbers: current number of gnomes killed, and total number of gnomes needed to be killed. For instance:
This means I’ve killed 21 + 205 + 200 + 195 + 190 … etc. which gives a total of 4326 (thanks for correction Grundulum!).
So the formula would be like… X + (N - 5) + (N - 10) + (N - 15) and so on, where N is the last goal completed (in this case, 205).
(Current progress) + (Current goal to kill)*(Previous goal to kill)/10
So for you, it’s 21 + (210)*(205)/10 = 4326.
Not 4296, unless I’ve made a mistake. 
Please tell me this counts as a polygnomial equation.
Oh you’re right – I had an error in my spreadsheet! Okay, thanks! That formulas work!
Why is it divided by ten?
You mean besides the obvious “so the math works out”? 
The formula for the sum of the first N numbers is N*(N+1)/2. But our count increases by 5 rather than 1, so multiply by 5. Since we want to use the gnome tracker numbers rather than mentally dividing by 5 each time, multiply N and N+1 by 5 each. Then we need to divide by 5 twice so that we haven’t changed the formula. Thus 5/(2*5*5), or 1/10.
Thanks for this. I’ll probably never use after this weekend, but thanks anyway 
Much thanks for the math @Grundulum!
For those like me who keep forgetting that the goal increases by five each time (… right?):
(Current progress) + [(Current goal to kill)^2 - 5(Current goal to kill)]/10
Now using variables, where
p = current progress
g = current goal to kill
p + (g^2 - 5g)/10
Apparently I killed 13,559 gnomes yesterday. 
Murderer!!
That’s why I can’t find them anymore, you genocide!
!!!