Harbeth 40.2 owner here. My experience is that open-topped stands are better than ones with a plate, and my preference runs to wood stands rather than metal. I went with custom stands from a local woodworker because everything commercial placed the Harbeth tweeter too high for my listening position. I’m aware that the Spendor tweeter is in the "mid" position, but allowing for that I still think you should aim to have it close to ear-height.
Other considerations: are you going to use something like Herbie’s footers or Isoacoustics Gaias on the feet of the stand? What kind of interface will you use between the bottom of the speaker and the plate/tops of the legs. I ended up using cork pads, but there are many options. Factor in all these, feet/interface etc., into your height considerations.
Dimensions: I’m guessing SA expect a slight overhang on all four sides; more elegant I suppose than having the speaker smaller than the plate it’s sitting on.