Graemevw,
I use a 4.6 mm foresight aperture for 6 yard & 10m shooting.
For me, the larger aperture works fine for shooting standing and unsupported.
I would suggest trying : 3.8mm, 4.2mm, and 4.6mm
and see how you like each one - that will guide you towards what is best for you.
Have fun
Best regards
Russ