Normally the ND (Neutral Density) filters are going in 1F steps:
ND 0,3 ; ND 0,6 resp. 1 F stop; 2 F stops so I think a N.D. 0,6 for 2 F stops is a practical choice.
With a TTL metering the correction is already there. Working with a seperate lightmeter simply compensate for 2 F stops.
Example: 1/250S f=11
--> 1/250S f=5,6
or 1/60S f=11
or 1/125S f=8
Best regards,
Robert
(owning a ND 0,6 Heliopan for his enlarger for doing RA-4 prints for a regular exposure time of 10-20S)
Can also match on the Leica M7 but the fastest shutter speed is 1/1000S.