I would go to the sketch and look at the thickness of the test plate used. What category does it fall into? 3/8 inch (10mm) or less, more than 3/8 inch (10 mm), but less than 3/4 inch (4.8 mm), or 1 1/2 (38mm) or thicker. That determines the range of thickness qualified for the base metal.
The thickness of the weld deposit qualified is a separate issue. Case in point: lets say you use a 1 1/2 inch thick plate (38mm) and you weld it with four different filler metals, each using a different process or F number.
Root layer (SMAW using F3) - deposit 1/8 inch thick - qualified to 1/4 inch max. with no minimum thickness.
Hot pass (SMAW using F4) - deposit 1/4 inch thick - qualified to 1/2 inch max. with no minimum thickness.
Fill pass (GMAW using F6) - deposit 3/8 inch thick - qualified to 3/4 inch (19mm) max. with no minimum thickness.
Back gouge second side to sound metal
Weld second side (SAW with F6) - deposit 3/4 inch thick - qualified to 8 inches with no minimum thickness
The procedure is now qualified for base metal from 3/16 inch to 8 inches in thickness based on the thickness of base metal used for the PQR. The SAW alone is qualified up to 8 inches because the deposited weld metal was at least 3/4 inch.
It would really throw a monkey wrench into the works if things were juggled so that F1 and F2 filler metals were used on the second side along with the SAW so no one process deposited a full 3/4 inch of weld metal. The procedure would no longer be qualified for base metal of 8 inches of thickness. The procedure would be limited to the thickness of the various F numbers used, which could easily be less than 8 inches.
I love ASME!
Back to your question. Converting mm to inches, I have a plate that is 7/16 inch thick welded with two welding processes. GTAW is used to deposit 1/8 inch and SMAW is used to deposit 5/16 inch. Rough conversions, forgive me if they aren't nuts on.
In my opinion, the 7/16 inch thick plate used to qualify the procedure would qualify for a range of 3/16 to 7/8 inch. The process GTAW would be qualified for deposited weld metal up to 1/4 inch and the SMAW up to 5/8 inch. The two processes together would allow you to weld the 7/8 inch maximum thickness. Either process individually could be used to weld the minimum thickness of 3/16 inch. There is no minimum thickness of the weld deposit, but there is on the base metal and it, the base metal, is the limiter for the purposes of writing the WPS. My answers are based Table QW-451.1 of the 2001 edition of Section IX. I have a newer edition somewhere, but it isn't handy.
Best regards - Al