A good rule of thumb for testing a windsor chairmaking mortise and tenon joint is if you can put the joint about half way together with only hand pressure then you are about perfect. Then during assembly you can force it togther with a few hammer blows.
In windsor chairmaking, you are not often putting multiple joints together at once so a tight fit like this is possible without complicating a smooth assembly. On the other hand, with a post and rung assembly, I will back it off just a hair so I can have a bit more control on an assembly. When you add up a lot of joints that have to come together at the same time on one of these chairs then the extra give can be the difference between success and a struggle or failure.
Here is a short (and very out of focus) video of how tight I try to make the joints for a post and rung chair joint.