Angle difference between 2 ERC coordinates

Seems like it should be a simple answer here but I can't figure it out.

I have 2 ecr coordinates e.g. (7.1747e+06, 3.0793e+06, 1.3263e+05) (7.1745+e06, 3.1209e+06, 1.3271e+05) and I need to find the angle between them.

Every solution I've found seems to suggest converting to another coordinate system, is that the only way? Feel it should be possible without converting.

Thanks in advance!

[Andy Wells]

After more searching, I've found the 'subspace' function, I'm going to work on validating it but does anyone know if this should work?