Scooterotica

I was told that 4.5 inches from the top of the legshields was the correct hight to

I fit them as high as they'll go, leaving enough room for the rubber surround. I supply them from scootrs and sterling and can't say I've noticed any differences. I like the sterling bare metal version as you can see that it's pressed [which I believe they all are] and there's no weld's or filler.
I now add my fitting method with each toolbox[on a document to save me re-writing it every time the question is asked].

I’ve fitted a few and this is how I’ve done them. I mask the area up where I'm going to fit the toolbox. Offer the toolbox up against the leggies, the higher the better but leave about 1/2 inch space at the side. Use a marker pen to trace around the toolbox that way you'll keep putting it back in the same position as you work on it. It's rare that they fit well out of the box so always need a bit of trimming to fit neatly. Wherever it touches is a high spot so remove a bit of material there. Do this from side to side and the gaps get less and less until you've got a neat fit all round. You just drill holes through your leggies for the 2 bottom brackets, the holes should be hidden under the mudguard. You just bolt straight through the welded lugs and through the mudguard. At the top the idea is to rivet the two supplied brackets to the centre section of the leggies, again the holes should be out of sight behind the horncasting. If you've got the leggies off the scooter it may be easier to use small bolts, about 4mm rather than the rivets. If you fit it with the leggies in place then you need to use the rivets. I glue the rubber surround in place using contact adhesive, start at the centre at the bottom and work your way round, taping the rubber in place with masking tape until the glue sets. Obviously trim it to size and dry fit it before painting or glueing the rubber on. I've only ever needed to use files and the worst fitting one only took about 2 hours to get the shape right. Only remove small bits of material at any one time, don't resort to tin snips and cut bigger amounts off to try and speed things up. A bit like having a haircut, you can always trim a bit more off but you can't put it back.