silly question about Loomis method

I want to reconstruct a face,, frontal. Using Loomis method to build the structure. If an eye is 13cm large, I put the center of the circle between two eyes so 13x3 = 39.cm. How much do I ad for radius of circle please ? I've been trying a few times but I'm not sure how to build it backward... is that silly of what ?!

(It's for the project I'm working on, you can have a glimpse on my profile pic 😉

I will be for ever greateful.