If have found that working with wood, the dimensions are not critical, the stuff moves/expads/contracts more than we realise, and things rarely fit perfectly straight away. If the wood stirp for the stringers is a tiny bit larger than needed in places, a quick tickle with sandpaper can make it fit perfecty. Just remeber that there are 4 sides/edges to the stringers. The outboard edge should be the one that is NOT adjusted. That way everything will stay true/inline. I always mark a face side with a pencil, so that I do not accidentally spoil it.
Sorry if I am teaching you how to suck eggs.