can you zip and upload your file?
You cant specify positions directly but you can drag your control points and they will snap to other reference points.
There is a short video in the SW help under Freeforms/Freeform PropertyManager/Control Points -about half way down the page- showing something similar to what you are doing.
Note it just snaps it doesnt form actual relations...
This non specific/non sticky nature of some surfacing tools is a shortcoming I keep prodding SW to 'enhance' and we live in hope.
Use direction 1 or 2 symmetry. Then when you create your control curve it automatically snaps to this plane. I would also use global for the triad orientation for the control points. I'm still on 09 so here is a simple example. I hope this helps.
FingerGrip.SLDPRT.zip 128.5 KB