I ask bikecad to tell me the correct dimension. Alternatively, you can draft it out or use trig, either is fairly simple.
I see nothing wrong with using paper templates as a starting point, although there are plenty of people that vehemently disagree.
First, I find the bow in the tube so I can point it along the plane of the bike. If nothing else, this helps when the customer gets out his finely calibrated string when he's trying to blame the frame alignment for his wonky wheel dish problems. Then I mark the centerline along the bowed dimension using marking fluid and a scribe. You can do it with just the scribe, but it's a lot easier to see with the marking fluid there.
Paterek seems to overthink some things, the number of people who have bought a surface plate and v-blocks staggers the imagination, but a method is there. And if you read about it, you can come up with a method that works in your situation.
Gavin, I have a Exel spreadsheet that converts center of tube to center of tube distance to edge of miters distance. Since the tubing centers are out in space you can't tell where they are but miter edges can be marked on the tubes. Then a mitering template can be placed on those markings. Email me and I'll email the spreadsheet back to you. You have to enter the angles, and diameters of all tubes (5 entries) and it will automatically give you the distance between the outside of the miters (where on the round they are closest together) on both the top and bottom of the tubes. Of course if your angles are identical then both top and bottom distances will be the same.