mt122 sketchup (not a plan request)

http://hornplans.free.fr/mt122.html

Obviously nothing fits the way i want it to when trying to do this, these are my first attempts.  My problem is I have drawn the flare from the flare lengths provided, and the 12 degree angle as marked is now 11.4degrees.  I originally started from the outside of the cab and found similar problems, so decided it would be better to draw the plans starting with the flare and modifying the exterior of the cab to fit.

I have a few questions: 

Should I draw the plan based around 12degree and 24degree angles on the flare and alter the lengths or should I draw the plan based on the lengths provided and use these slightly off angles?

How critical is the volume inside a flare? If none of the angles/lengths quite match up surely I will never find the ideal volume?

Am I approaching this the right way?  I notice everyones mt122 builds are slightly different, is it a case of just going with what i think would be the best compromise?