hi graham! the way i did the deck markings was to blast a broad white stripe end to end, then using a metal ruler i cut a strip of tamiya tape to the width i wanted, then i measured across the deck at three points (stern, middle and bow) then divided each of those measurements in half to make sure it ran stright down the middle. with the line of tape in place i then sprayed the deck green (at this point i wasn't going to put the circle marking on) i then had a change of mind, and sprayed a broad circle onto the deck, like so

i then used a compass cutter to cut first a circle, then a ring in a piece of frisket film, stuck this down, after drawing a line bisecting it, again to keep it in the middle, sprayed the green over, and when all the masking was off that was my end result.
i'm sendin a pm about the roundels, that would certainly save me a job!
cheers, phil