Where Impact Testing is required, be very careful with the MINIMUM qualified thickness, in your instance, if the PQR is 7mm thick, the minimum thickness of the WPS is 7mm (See QW-403.6) NOT 6mm however, if your PQR is 6mm thick then yes, you would be allowed 1/2t or 3.5mm as you indicate.
may i know u are intend to
case 1) qualify a WPS to weld 6mm to 32mm (same joint different thickness) during production or
case 2) have a WPS just for the range of qualification from 6mm to 32mm?
if case 1, please refer to 202.4 (b-1) for some P number, u only need a qualified WPS base on 6mm.
that's means only PQR tested at 6mm coupon with charpy-v impact can be used.
if P number does not fall into 202.4 (b-1), u will need to follow case 2).
if case 2, for sure u will need at lease 2 PQRs with charpy-v impact tested,
1) 6mm to qualified from 6.0mm to 12.0mm, as what jon20013 has mentioned 7mm does not qualified 6mm when charpy impact is required.
2) min. 16mm needed as to qualfy up to maximum of 32.0mm