That's surprisingly close to my calculations. The numbers that I use are:
Coffee 1.21
Gold 1.46
Diamonds 2.05

So clearly I am a little more relaxed with the buy/sell points. After playing the stock market for over a month, I just don't have the patience to wait for the perfect price.

Also, I use "D -> C -> G -> C -> repeat" only when I am not particularly busy with anything else. Most of the time, I just check the game every now and then and either "sell Diamonds/buy Gold" or the opposite. I do it way more than 8 hrs/day though (I am around my computer a lot).

It's a little disappointing that the levels max out at around e300 - this does not give enough shares to max the sign decoration. So far, I maxed one girl (#11, which is apparently the cheapest to max). I will try to see if it is possible to buy others past their maximum - by setting the buy option to "max levels to buy". This method allows to buy past multiple upgrade points without actually buying the upgrades. But I doubt it will work the same way here.

If someone has the maximum levels for all the girls, please post them. They are a little tricky because once you reach that level, the game does not tell you anywhere what that level actually is (which is really stupid). So I don't know what my girl 11 currently is, but I have a way to calculate it (Total level ups minus ten other girls levels).