bobble-
with Steel and Alu guards the way you adjust the guards to fit the radius of the tyre for better fit is by either squeezing or spreading the guard.
Squeezing the guard will increase the radius
Spreading the guard will decrease the radiusFrom the factory they rarely follow the radius of a wheel perfectly, so some adjustment is needed. You can often do a bit more int he way of manipulation to get a bit more/less clearance as you desire too.
With plastic guards you'd either do this with a heat gun or pull them into shape as best you can with the stays, but if you do that on metal guards they'll be sitting under tension and will crack a lot sooner. You have to adjust them to fit properly without using the stays to get them in shape, the stays should literally just do that, keep the guard mounted where it's placed without holding them under tension.
With the 38mm VO guards I spread them to about 40mm along most of the guard, gave me a bit more clearance width wise and to follow the tyre properly with an even gap around the tyre.
Not my site, but this guy did a good write up of the procedures:
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NotDotMatt
When using 700C or 650B?
If you're using 700c then 45mm will fit through at the rear with a bit of manipulation/crimping at the BB area, but I think you'd struggle to get the 45mm ones through the fork crown without some serious crimping!
Mine is using VO 38mm (with a bit of spreading to about 40mm while keeping the radius in check) for 700C and gives adequate clearance with 28mm tyres.
If you're going 650B then the 45mm ones do work as the ~10mm drop for where they guards will need to sit brings them down to a slightly wider bit of the fork.