Impact flyers unite -- where is this damn horizontal reference line??

On mine, Andy P's approach would have worked as well as the approach I took (and been quicker!!). The stab tube/anti-rotation pin marks on the fuz were close to the consistent with the datum line I ultimately used.

For reference, the datum line I used runs from the flat bit the engine cowl sits on to 30mm below the centre line of the stab tube hole. On the pre-painted model this is the bottom of the blue stripe (with the caveat that there is about 3 to 5 mm difference in where the paint line it is on both sides of the plane). Assuming all the Impacts are coming out the same mould/moulds, and that you accurately open the stab tube and anti-rotation pin holes, that would be the quickest way to get a sensible result. Alternatively draw the line from cowl mount flat plate to 30mm under stab and use that.

A couple of other thoughts from Mark and others comments:
1) The one thing I didn't trust on mine was the alignment of the front of the fuz (whathever that bit is that the spiinner backplate sits relative to) -- mine has about 2.5 degress down thrust in it (Mark, if you hence used it as a refernce for setting the fuz up, it would be the source of your error). It is not the recommended 0 degrees incidence starting point. I will end up with uneven gaps between spinner backplate and fuz.
2) Right thrust at the front of the fuz was closer, but the holes pre-drilled in the firewall are more consistent with about 5 degrees right thrust -- they were 12+ mm off centre rather than the 6 ish required for 2 degrees.
3) I am not sure on the fin alignment -- I have had a couple of looks and I think it may be out a little, but given light and reflections etc. I am not absolutely sure. I will check later -- along with possible wing twists.
4) I did try to see if I could keep the stab anti-rotation pin in one piece -- making the stab mount more rigid. Ultimately I cut it in two as the retainers for it in the stabs in mine were not quite aligned -- and it made getting everything squared up too hard

David