Gardengate Manx / Inter rear wheel offset

so I  roughly laced up the wheel and then trued and centered the rim while installed on the bike using a string running from the steering head over the seat downtube. Close enough for jazz.

…. my machine; will measure up tomorrow for you.  It’s will be based on complete wheel fitted in machine so not as accurate as during build phase.  
Jon

Viewing from the rear of the machine the hub nominally 4.75” wide at the axle has spoke flanges nominally 3.25” apart.  These appear to be on the same centre line. The hubs spoke flanges are different, the left side closest to brake drum is almost vertical the opposite one has an inclination inwards. 
The wheel rim is 2.75” wide. The rim sits off the centre line of the hub to the left (looking from rear) by 0.435”

What I give you is the dimensions of my configuration.  What your goal is may well differ.

The basis of wheel offset is to set the rim on the centre line of the frame. The frame centre line dictates the position of the gearbox as it is mounted on that central lug on the seat down tube.  That then gives you the position of the drive sprocket. That in turn needs to line up with the final drive sprocket. This then determines the offset required for the wheel to bring the centre line of the wheel in line with the frame. 

The check is to measure the offset from the back of the brake drum (translated to the face of the wheel hub), to the rim edge. An easy one with a straight edge on the back of the drum extended out,

Mine measures 7.3mm, my mates rigid version measures 5.6mm

Spoke length calculations are available on line.

Hope this helps.  I’m no expert but my configuration seemed similar to your needs.

Cheers

 Jon