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).

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.