Hi Neil.
First off, don't get the willmann-Bell screens, they count the lines wrong and are 1/2 what they state. Get them from
http://ronchiscreens.com/
With your tester, it's a bit hard to tell from your pics but the light seems in the correct position with the knife edge cutting 1/2 way over it. The return beam also needs to be cut into with the knife to get the shadow (above the light). Start say 1/2" inside where you think the RoC is and there should be a dark, sharp shadow coming from the same direction as the knife. Move the knife back until the shadow forms the doughnut pattern. Continue moving the knife back and the shadow will come in from opposite the knife.
A possible problem I can see is you need to have your eye right in close behind the knife edge which might be difficult with your set up.
Dave