Use this bit of code to calculate the scale. It is obviously dependent on the distance to your perspective camera.
float zdist = (tilemap.transform.position.z - Camera.main.transform.position.z);
float s = tk2dPixelPerfectHelper.CalculateScaleForPerspectiveCamera(Camera.main.fov, zdist);
And set s to the tilemap scale, and thats it.
You can also get the scale by dragging a sprite from the same collection used in the tilemap, position it at the same z distance as where the tilemap is, 1:1 it, and use that scale value on your tilemap.