Good stuff -I like the model idea to help us understand what we are talking about.

I dont know why the gross profit for the year is the monthly number X 6 - s/b12??

Then I am also not quite sure what your objective is here?
profit margin that makes the working capital increases self financing
If the example has a growth through a predetermined percentage turnover number then the working capital will be a balancing number. If we work from the working capital side using the additional cash generated and you want a percentage of that to generate turnover and grow the business from that angle then we look at it from the balance sheet side. (This is what I assume you are after). I think it could get complicated if we tackle it from both sides.

Hopefully I have not confused the whole issue and put you back to square one.