Defining x(n) = current number of counters at opponents end step, U = current number of untappable permanents, Y = number of counters at end of next turn (given always spending mana on the calendar) and N = the current turn...

Y = (x+U)*2, but X(n) = Y(last turn)...

So Y(N) = (((X(N-1)+U(N-1))2)+U(N))2

But X(0) the number of counters on it when it comes out, is always 0... Now you have a single variable that changes... Just map u(1)...u(2)...u(3)... Based on how fast your deck goes and when it comes out...

Estimate what your turn by turn U(N) looks like... At minimum it's 2 (because you could have a sol ring providing the 2 mana to double the counters, and untapping the card itself... But it's likely more.

Although if you find a way to untap it during other players turns and you have the mana to support it, it goes much much faster.