Xtr 970 has an inner ring which is way thicker than the steel SLX one i am about to replace it with. Any ideas on how many mm of spacers to add. Or just line both up and add by eye. Intend to use disk shims.

W,

Measure the middle-outer spacing and go for the same for the inner-middle.

Disc calliper shims are for 5mm bolts, chainrings are 8mm bolts so I think you may have some dremelling to do!

Normal 8mm washers of approx 0.75mm thickness will probably be near enough.

PaulD

2mm is the spacing you need and rotor shims won’t fit. You need these:
http://www.pro-bolt.com/aluminium/aluminium-nuts-washers.html?cat=778&thread_washer_dia=164

Choice of colours too!