A* graph search heuristicfor pathfinding
There is nothing wrong with the heuristic function you suggest. Note what the consistency condition actually says: If you have nodes $n_1,n_2$, and the cost of traveling from $n_1$ to $n_2$ is $c$, then the heuristic values need to satisfy a triangle condition: $h(n_1)$ must not be more than $h(n_2)+c$. It is ok for $h$ to underestimate the distance from a node, as it does for your start node (though obviously, the performance of the algorithm can suffer in this case).