carefull hombre, i was getting tied to trees a few days ago , by the neck , because I opened my big mouth/fingers on keyboard,
When it comes to me and nucs I only ever see them as short term accomadation and no more , bees I have installed last year , June will only stay in there till aprill/may this year and thats the far limit, some times things go to plan in which case its july/august till aprill some times a little longer but never over the 14 months mark, There are some people who use the nuc box all year round either swapping bees or queens. Why do I say this ? because it effects the answers. I am short term and as such do not have to worry about brace comb build up. Also I only use cheap and nasty ply/timber and can only work to within a few feet let alone millimetres when dealing with wood so for my own use a gap of around 15mm is fine as if any thing does drop, dead bees etc there is still room for bees to move about any thing larger than 15mm will get you brace comb build up