what pinion
 

What spur and pinion should I use on my dex 210 with a 10.5 motor?

What sort of course?

Worksop masters so usually pretty quick

I've no idea what worksop track is like.
Big, small, bendy, open, tight, carpet, wood.........

But the starting point with a10.5 on stock spur (83?) would probably be about a 25/26.
If it's an rtr (78 spur) maybe a tooth less. Then adjust by motor/ESC temps and performance. THEN start mucking around with boost/turbo (if you are allowed it)