I invented my own little method to calculate a "realistic" GDP for our nations. Basically, a formula converts the economy value of a nation into a "growth" value. Your nation has a starting GDP of "100", this is just a base, what it was before you took control, and can be whatever you like. But after you take over, the gdp is calculated every day and changes by the growth value.
If you economy is 0, the growth is -0,1%. If your economy is 40, it's 0%. If it's 100, it's 0,15%. So if your GDP was 100, and your economy in NS is 100 for the next day, your GDP will become 100 + 100 * 0,15 / 100 = 100,15.
This also means growth is exponential. Over an entire year, you can get a probably unrealistically high growth... If it stays at 100 for a year your economy will grow by 73%. It would fall by 31% if it stayed at 0 for an entire year. The formula could be made more complex and changed, but I thought that 0% growth should be around 40.
Here the data and graphs are for Blaist Blaland. I can make similar graphs for your nations as well if you like, but max 2 nations pr. person.