Been meaning to do this for a while a little aid to calculate the exposure offset when using ND filters covers ND2 to ND1024 (1 to 10 stop)

I have made the print to fit a CD/DVD if you stick the btm one to a CD and the top to the clear one you get in packs of blank CD’s it should work ok, or make any size you want and print onto stiff card.

The aperture and ISO are not adjusted only the shutter speed.
Take a meter reading off the camera noting the shutter speed
On the calculator the outer ring contains the metered shutter times the inner ring the adjusted shutter time with the ND filter in place and the ND filter strength showing in the window.

Right click on link and use save link as…      Download jpeg