After more research and asking about, I've learnt that this will use a series equation..
That's as much progress as I've managed to make so far on this.
But to try and make it easier to understand what I have to do i'll try another example.
Say the user gains 1 population per second, the gold income that he would have would be the case of over the course of seconds...
1+2+3+4+5 up to the capacity of the population, so if it capped at 5 population over what was currently there then it would continue as 5+5+5+5 etc up to the end of the range.
I'm so terrible at trying to put things into explainations, so I really hope that this part with the first post makes more sense!