I recommend every guitar builder should have a centre ruler, they are perfect for the job. Mine is from StewMac but Crimson Guitars have them as well. 
Crimson – https://www.crimsonguitars.com/products/crimson-guitars-precision-centre-ruler-100mm?pr_prod_strat=copurchase&pr_rec_pid=4571259863175&pr_ref_pid=4571257307271&pr_seq=uniform
StewMac – https://www.stewmac.com/luthier-tools-and-supplies/tools-by-job/tools-for-measuring/stewmac-shop-rule.html