Fire Door Threshold Gap

[gor81]

Hi Everyone

Looking for  a viewpoint on the following if anyone can assist please.

Threshold gaps to fire doors should obviously be kept to within permissable limits and if a door performs a smoke stopping function then 3mm is regarded as the accepted threshold gap. 

To reduce such gaps where threshold gaps are becoming excessive then an accepted practice would be to add a hardwood strip with a minimum of 6mm to the bottom of the door by glueing and screwing. 

Question: Is there a maximum size put on any timber hardwood addition to reduce a threshold gap at the bottom of a door?

Thankyou for any help in this department.