AE82 4AC Timing (#) BTDC or ATDC - (Dual Vacuum)

Got a problem with the Haynes service manual.

It States, set timing to 5 degrees <>2 deg.
but no if it is before or after.

Got a dual vacuum advance unit.
It runs bad on BTDC and ATDC. No telling yet.

any help.?
Old thread, but timing will always be BTDC, and you are supposed to remove and plug all vacuum lines at the distributor advance while setting the timing at engine idle speed.