Happy to hear that you like it :-).
And yes you're totally right. I should have increased the bonus-time. My calculation was done for forward and back + a little extra time. But if you've to do one turn you loose time. I wanted the player to loose time during the game but this is too much because it is nearly not possible to build up time.
My goal was to make it hard to reach the (afair) 6th level. Because from then on I don't add any rows anymore. I think I've simple overshot.