The detailed instructions you quote are not mine, but Bill Rouse's, and they are a good detailed description of the assembly sequence for the C-drive seals.
Remember that the bushing turns with the shaft, and the seals do not. The key is you want the face of the bushing (or as Amel calls it the "wearing out bearing") to NOT touch and rub on the the outermost seal, AND you do not want to pull it so far out that the deepest seal drops off the inner edge or to where the aft side of the bushing interferes with the line cutter. Also, a smaller gap here between the bushing and seal reduces the chance of fishing line, or seaweed wrapping on the shaft and damaging the seal lip.
I do not measure the 0.5mm, I just make sure I can see a crack of daylight all the way round, between the shoulder of the bushing and the seal. As my grandmother would have said in her own highly technical style, "a schooch."
Bill Rouse might want to throw in his ideas here as well, but I think we are talking here not about 0.5mm +/- 0.01, but rather something more like 0.5mm +/- 0.2 In other words, if you wanted to be totally compulsive about it, a feeler gauge would give you an exact answer, but it is more than a bit of unneeded precision.
Spanish Wells, Bahamas