Hey Martin

... but just in case you have nothing to do and want to offset the rocker arms, just to have it look out of symmetry, or just be be scale, or just for the heck of it...

According to a report in the Flight Journal: THE 230 H.P. BEN2 AERO ENGINE., Notes p204-212,
Magneto type, 2 Bosch Z.H.6; firing sequence of engine, from the prop: 1-5-3-6-2-4 ; numbering of cylinders from the prop back: 1-2-3-4-5-6

A couple of diagrams to help, looks like there may be alternative set-ups (see 2nd diagram)
Hope this helps,

Brian