From knife making the standard has always been for me that it is tight enough to hold the blade when it turned upside down but can be removed with a pull of 2 fingers on the blade and 2 fingers holding the sheath.

Any looser and there is a change of a blade coming out of the sheath at the wrong time. Any tighter and you will not only scratch up the blade more than needed but also the use of excess force when pulling a blade is bit dangerous.

Jim