Space same as intended clipped height, so 1-1 1/2 ft. apart in this instance. Think of each as a block in a line of blocks, and how close you will have to put the blocks together in order for them to touch.

Of course, since this is an informal planting with a curved bed shape you shouldn't actually be planting long rows of one kind of shrub that is being kept clipped...

Or "stripping" the bottoms of those other shrubs at the one end nearest the car in the second shot, so that it looks like you have a deer problem. And maybe feel therefore the need to fill in with the box, due in part at least to the artificially created lack of meat on the lower halves of the existing taller shrubs.

I don't know if there are different varieties of boxwoods, but some of them are not particularly happy with our cold winters. For their first year, you might consider putting burlap or some other wind break around the whole border to help them handle their first harsh year. I suspect they'd be better once established, but I'd err on the side of caution since you have so many of them.

My mother kept some Southern boxwoods alive in Michigan for many years, with this kind of protection for the first couple of years. Once she stopped the protection she got away with them subject to the winter winds on their own, and they survived with some lopsided dieback every year. The more you want them to be symmetrical, the more attention you may want to pay to a wind break over the winter. Like our roses, I wouldn't totally enclose them in anything to avoid moisture problems

Of course there may be winter hardy boxwoods that don't need any of that treatment in zone 5, but it's worth considering.

Cynthia

Hey... they are the same kind that I have in other areas of my garden and are supposed to be really hardy even to one zone lower than mine, I never had any problems with them in winters before, other than trimming too late in the season and new growth nor hardening enough, but thank you for the recommendation anyways!