I would assume pretty accurate, if you are using only iron, but I'm not sure since you're using diabase. I notice that their disclaimer mentions not to use their calculations for alnico materials, and I don't know what those are.

More to the point, however, is alignment. The magnetic field strength that this website outputs is what you get at a POINT a certain distance from the top of the magnet, and even then only if that point is centered PERFECTLY with the axis of symmetry of the magnet. Any deviation from that point - or, for that matter, if you have an object that you are measuring that is larger than a point - and the magnetic field will change, becoming weaker. As such, in order to get the closest results to the website output, you need to place a very small piece of diabase away from the magnet, small enough that its size relative to the overall shape of the magnetic field can be ignored.