This is one that really needn't be written into the SOPs, it belongs to the basic application of performance principals as applied by operating crews.
If your MEL specifies a Performance Penalty, then there's 2 actions which must be taken -
(1) Ascertain the normal RTOW, and SUBTRACT the performance penalty to find the "new" performance limited weight, and
(2) If your actual weight is below the "new" limit, ADD the performance penalties to the actual Takeoff Weight to find the "artificial" weight for which you will calculate Reduced Thrust / V-Speeds etc.
Sometimes you'll find 2 limits to apply, one for the Climb limit, and a second for the Field/Obstacle limit. Apply both in both cases stated above, and fly at the most limiting.
You asked the question for the B767, but the philosophy applies similarly from the DC3 right through to the A380
Regards,
Old Smokey