James and I did a very rough measurement of the target foil z. Not very accurate, but just to rule out any gross mis-position:
from face of upstream post mount (figs a1.png, a2.png,a3.png) : 23 9/16 " from face of upstream bellows (figs b1.png) : 40 15/16 " from PVC coupler (fig. c1.png) : 16 3/4 "
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5