Planning an application is exciting enough. So don`t worry about the ring size. This can be adjusted after the application to the finger of your loved one. If, on the other hand, you want to make sure that the ring fits after saying "Yes", take one of your rings in an unobserved moment and mark its circumference on paper. This will give Lesunja a good indication of the correct size.

White gold 750 18K half-eternity ring with 1 diamond 0.17ct. TWsi and 12 diamonds 0.086ct. TWsi
Ring size: 54