Well, from experience I can honestly say you will never get them perfect, unless you want to spend megabucks buying the motorized ones. Not only would you need to get them in the exact same spot but as you clamp down on the mirror housing, it will flex and never flex identically. As a tip for general use, the further out on the mirror housing you put them, the less stable they will be as you are adding to the fulcrum effect of the arms themselves. Add that to British roads, tyre vibration and wind speed and it all makes for an unstable mirror. Another tip, from an ex-trucker and Im sure Parksy will agree, is in very wet or misty weather, tie a strip of rag about 2 inch wide and ten inch long next to the mirror on the mirror arm, and it will flap around and continually wipe your mirrors.
Finally, for those stuggling with a copilot that finds the mirrors resist any attempt to adjust them, and then all of a sudden jump a couple of inches, there is an answer. The mirrors are usually on a ball socket, and they get dry and tight inside. I have also found that Milenco in particular very often have a tiny nub on the ball from the moulding process. This nub can easily be removed by a stanley knife or some fine emery cloth. Its easy to flick the ball out of the socket and you can then apply a thin smear of grease to the ball. Handy hint, either copper grease or as I use, powdered graphite. To flick them out just push the arm as far as it will go back on itself, than add a tad more pressure. They will pop out without damage, just needs a stong thumb or hide mallet to persuade it back in afterwards.