I can't seem to find this question answered anywhere. I have Ed. 2 of Ianto's book and there are conflicting dimentia: the tunnel in written suggestion as 1/2 riser height but in a sample combustion unit sketch the riser height is measured (the arrow span) from the bottom of the tunnel whereas another sketch shows it measured from the top of the tunnel. This is a big difference.
What is best practice and specifically from where are the dimensions measures for riser height?
There is not an official standard for this. Various experts use different criteria. I like Peter Berg's "centerline" method, using a 1:2:4 ratio with "1" equaling somewhere between 0.7 and 1.3 x the system diameter... this as discussed I think in a thread on the donkey32.proboards.com forum. So the whole thing can be a bit bigger or smaller for a given system size. The bigger the combustion core proportions, the stronger the rocket effect. As mentioned in that thread, the "outer dimensions" method (length of longest surfaces) is alleged to be easier to apply if laying out a system live with bricks, while the centerline method is easier if laying out on paper/cad and especially if casting a core.