Since you have a half scale model to work from you might want to buy a center-finding rule, center-point rule or centering rule. They are used for finding the center of measurements. There are several different styles, but one is a rule with regular 1" markings on one edge and on the opposite edge a 2:1 compressed scale.

Model makers also use tape measures with scaled markings on them. I have one 3/4"= 1'-0" its basicaly marks every 3/4" with 12 subdivisions. Don't pick it up by mistake, my wife tried to use it to measure for miniblinds! You may be able to find a tape measure with a 2:1 scale.