OK, here we go.
Take at least 3 balls from the paint you intend to play with, this is because there may be a very small deviation in size through the box & you really don't want to base your insert choice on the one aberrently small/enormous ball from the whole batch.
Take one insert, around .687/.689 is usually a good start if you're using Blaze. Try to drop each ball in turn down the insert. If the balls roll down freely then the insert is too big. If they lodge in the insert then try blowing reasonably gently down the insert - the ball should then come out the other end. If you find you're having to blow very hard & are starting to go red in the face then the insert is definitely too small. Do I really have to say what happens if the ball won't fit in the insert at all?
This should give you the right insert for the paint you are using BUT if you change batches during the day then recheck your insert size before playing with the new batch, e.g. from red/yellow shell to blue/orange shell = different batch.