Have I nearly found the event horizon of a black hole? The calculator on the website https://planetcalc.com/1758/ cites Wikipedia's Gravitational acceleration and implements:
$$g = G \frac{M}{(R + h)^2}$$
For a black hole with mass $M$ equal to 5 solar masses with a diameter $R$ of 1 meter at a distance $h$ of 1500 km, the acceleration given is 296000 million m/s^2 which is nearly close to trap light.
So one might expect the event horizon to be at least close to that. Is this correct?
This is the Newtonian model of gravity. It is a very good model, it is used for accurate calculating the motion of objects in the solar system to a very high degree of accuracy.
However, for very strong gravitational fields you need to use Einstein's model, which accounts for things like the constant speed of light for all observers. I'm not going to go into mathematical details (not least because they are far to hard for me!)
Now you can calculate the gravity of a sphere and you find that at a radius of $r=2GM/c^2$ something strange seems to happen, nothing can travel from a radius of less than this to a radius of more.  It is the event horizon.  This can only happen in Einstein's theory of gravity.  Newtonian theory has no &quot;speed limit&quot; so there can be no event horizon.
Now it so happens that this is the same radius that Newtonian gravity would predict to have an escape velocity $v_e = \sqrt{\frac{2GM}{r}}$ equal to the speed of light.  That is mostly a coincidence(there was a recent question about this, that I haven't been able to find), but a neat one.
You seem to be comparing the Newtonian acceleration due to gravity with the speed of light.  The units of acceleration and speed are different, so the comparison is not valid.