Itank it is normal that the tire does not center into the frame, and that is because of the caliper on the right side...
As long as your belt is not rubbing the left side of the tire, and your tire isn't in contact with the left side of the fender or your right side of your tire is not in contact with your right side of your fender you are good to go.
It does NOT tract within the fender in a center position.
You could always take a caliper and measure the difference between both distances on each side of the fender and calculate adding more length to say the right side spacer to see if that would give you more space on the right side and less space on the left side. You would also need to make a thiner spacer to add to the left side....i would leave your OEM spacers alone and have a machinist start from scratch as to what you wish to add to the right and remove from the left as then you have a reference point to go from.
But as I have said the reason it does not track in the fender from a center position is because of the brake caliper on the right side.
All the narrower rear wheel sprocket allowed for is more room for a wider tire....
Just make sure the width of the tire isn't going to hit any bolts/nuts on the inside of your strut, which means that you might consider reversing said strut bolts using button head screws to allow for a smoother finish inside the fender and using more decorative nuts on the outside of the struts if you reverse the direction of the bolts from inside to out verses outside to in, if that makes sense....
I am running an AVON VENUM tire on my rear right now that is about a 143mm wide even though it is suppose to only be 140mm wide tire, and really it is wide enough....and that took some work to get it in there properly but that is also running the OEM 1 1/2" belt..
I bet it looks "visually" like eye-candy to be able to get that 150+mm (depending upon actual size) stuffed into that rear fender....
Regards,
Tim