Partial Pen Butt Weld

Hi Lads,
I need some help with writting a procedure for partial pen welds with regards to the thickness range of the material.
The pentration of a part pen butt weld according to 1554:2004 is depth of bevel minus 3mm using FCAW with a 45 degree bevel. Now, does  the thickness range, which is an essential varriable, based on the depth of bevel or the thickness of material? 1554 does not seem to be clear on this fact as a full pen weld is the full thickness of the material so the thickness can be based on 0.75 and 1.5, however a part pen is based on throat thickness. I hope you can help!