Got some fox 34 forks and I want to use a 203mm Rota with xt calliper , so what adaptor do I need , cheers

I had this myself recently albeit a different fork.
Basically find out what your fork adapter lug size are(mine were for a 160mm Post mount caliper)then order size adapter that fits.
Is it a post mount fork or I.s?
I’ve not properly explained.Sorry.
I purchased from uberbike parts for my 203mm adapter,there are all sizes whether i.s or pm.

Are you using the fork (and brake already) with a different rotor size (guessing 180mm) ?

If so, and the fork is post-mount and there’s NO adaptor already between the fork & brake, then you need a post mount +20 adaptor (front). Google will find you loads, then you need to find one in stock

The 34 has a 160mm post mount fitting, so if you want to use a 203mm rotor you would need a suitable adaptor. Like this one:

https://www.ebay.co.uk/itm/Shimano-Disc-Brake-Adapter-Front-SM-MA-F203P-P-PM-Caliper-to-PM-Fork-203mm/191592792443?epid=1905115782&hash=item2c9bd2017b:g:~~4AAOSwBLlVbLtH

If so, and the fork is post-mount and there’s NO adaptor already between the fork & brake, then you need a post mount +20

Yer math’s off. 203-180 is +23mm
Only 36 are 180.. (pm7) 34 are 160mm, (pm6) .