Thanks for playing.
The area is filled by flood fill method. The Fill force is the iteration on with the current tile was filled (or the distance from the dice tile without moving diagonally). I can get why it can be confusing since it was not explained. (I don't even think I explained it properly now, that's why it was not in instruction)
I am happy to know the game look polished. In my previous Jams I have been told that my gameplay was to complex and there were to many instructions. So this time I went for a really simple mechanics (even a bit underwhelming), and decided to see how polished and good looking I can make in in 2 days. Good to know that there was a little bit of success in that area at least.