Hi,
I think I have found a very simple method to set head tubes on a straight front boom. Get two matching ladies bikes with matching top tubes and matching head tubes (that slope downwards - mens top tubes are typically near to horizontal). When the ladies top tubes are held horizontal their head tubes should be at about 12 degrees. Slip the ladies top tubes into a slightly oversized pipe (snug fit) with boom length (oversized pipe) to your track requirements then weld (where head tube touches end of oversized pipe) or even bolt through both oversized pipe and ladies top tube within. If bolted there is the possibility that the track width could be altered at a later date by slidng top tubes (in or out) and adding appropriate drill holes to oversized pipe (Ackermans may have to be altered ). Attach steering boom (oversized pipe) to main boom so that head tubes are at correct caster. Done. Regards..........