Smokebox dimensions

Have been working out dimensions for the chimney liner and blastpipe/blower arrangement. If you stick to the 1 in 3 and 1 in 6 rules (see JvR's diagram above) it presents a challenge with such a short protrusion of the chimney over the top of the smokebox. With a 13mm ID liner (i.e. a bit of radiator pipe!) you end up with needing to put the exhaust jet underneath the smokebox which clearly isn't practical. Lining the inside of the chimney to 10mm means that you can just about get the exhaust into the smokebox as it can sit 60mm below the top edge of the chimney. I might ask a friend with a G1MRC Britannia how it works on their one as they all seem to steam very well. Perhaps it uses some kind of multiple-jet arrangement? Once I have this sorted I can get cracking with building a proper smokebox and try the boiler on the blower.