A* Admissible Heuristic for die rolling on grid

Question

I need some help finding a good heuristic for the following problem:

You are given an R-by-C

Answer 1

Idea:

If you have to move in a straight line, the best you can do is to end your moves with 1 and 2, for all other moves you can't do better than 3.5*distance.

Heuristic:

With ManhattanDistance = x + y the following heuristic could be used:

Heuristic = xH + yH;

where

xH = calculateStraightLineHeuristic(x)
yH = calculateStraightLineHeuristic(y)

and the function calculateStraightLineHeuristic(z) is defined as follows:

calculateStraightLineHeuristic(z)
    if (z = 1)
        return zH = 1
    elseif (z = 2)
        return zH = 2+1
    else
        return zH = (z-2)*3.5+2+1
end