I would say "When you are ready, please assume the start position," if they don't assume it after a reasonable period of time. They generally either ask for more time, or ask what the start position is.
If someone gave me lip about breaking their, concentration, I would reply something to the effect, "Would you like to continue or would you like to unload and show clear, and you can shoot when you are ready."
Life is too short to let ignoramuses get under your skin, just be polite and move on to the next shooter who understands the COF and the start position. Maybe they will pay attention during the walk through next time
As a stage RO for a big match I always demonstrate what is accepable during the walk through (for the sake of "trouble makers") Less headaches demonstrating it.