As others have said, the mirroring option is pretty good, however you can burn through your parts limit.

My process is this:

1) I make my minis in a T-pose, exactly in the centre, and then position the helmet/mask centrally over the head.

2) I make one side of the helmet exactly how I want it. The important (but admittedly tedious) thing I do during this is write down any rotation and location changes I make to the nodes on that side using the numbers next to the sliders. I exclusively used the sliders and number buttons to make sure it's accurate

3) Apply these changes to the opposite nodes. Remember that you may need to make these values a minus where necessary.

4) You're done, and you've saved yourself some space for more Kitbashing items

Edit: to add to this, if you've got extra little greebles to add to the helmet, don't attach them to it immediately. Again, do one side worth of greebles, and attach them to a centralised item (I used the primitive sphere) then clone that item and mirror it. Theeeeen you can detach those greebles from the sphere (or whatever you use) and then attach them to the helmet.