Focuser Baffle Question

Hi. I'm building a 16" f/4.5 strut dob with a single-ring upper assembly. There will be a baffle on the opposite side of the secondary from the focuser. I've read that another baffle between the focuser and secondary, just behind the bottom reach of the focuser, would be helpful. How do you figure out the hole diameter for that baffle? I'm thinking it would depend on how big of a fully illuminated field you want. Is that correct? Can anybody give me some tips on how to figure the baffle hole size out? Also, if anybody has recommendations for the size of the fully illuminated field that I should design for, please provide those also. Thanks!