The standard 6" BP that comes with the Rupes 21 Bigfoot (#981.321) is 5-7/8" (149mm)
This BP would give you 3mm all the way around the Velcro with the 7" HD-Orbital Pads
To run the 6" pads, you would need to pick up a smaller backing plate (#980.027) for the LHR15ES Bigfoot which is 4-7/8" (123.8mm)
This BP would give you 2.5mm all the way around the Velcro with the 6" HD-Orbital Pads
7" pad Velcro is 155mm
6" pad Velcro is 129mm - 130mm