Thanks! I did try. The way I do that is so:
Value of piece it's changed to - value of the original piece + 3 + (20 - turns taken)/2
And those are the values: (Queen = 12, Rook = 5, Bishop = 4, Knight = 3, Pawn = 1).
And the last part ((20 - turns taken)/2) is constrained to be above -1.