I found that a leather washer coupled with the 4 mm transfer port had so much lost volume that the piston slammed into the end of the cylinder.
I fitted an O ring piston head and increased the stroke on my 35 to reduce the effect of the lost volume caused by the 4 mm transfer port.
This is how I ended up at 15 ftlbs.
When I destroked the gun down to 12 ftlbs I ended up with piston slam if I used anything other than proper Wasps.
I really must build my 35 up with my spare 2.8 mm transfer port cylinder sometime for comparison with the 4 mm cylinder.
All the best Mick