Hi - really interesting format. With .22LSR 25yd/m prone shooting we have a sighter card (to the left usually) which can be used to get settled and check dioptre sights but I think the main use is to get the barrel up to temperature (and the breech flag helps keep it there).
Do you have sighters? How many shots permitted? Sighters any time? I recall seeing some BR clubs having fairly detailed rules on sighter use but that may have been full bore on those super duper hydraulic sledges :-)
I assume the cards are signed before the start and witnessed afterwards - the Evil White Sticker of Doom which turns a confident shooter in to a nervous wreck :-)