1/60s exposure without filter:
ND2 shutter speed = 1/60 * 2 = 1/30s
ND4 shutter speed = 1/60 * 4 = 1/15s
ND8 shutter speed = 1/60 * 8 = 1/7.5s round up to 1/8s
ND400 shutter speed = 1/60 * 400 = 6.667s around up to 7 seconds
assuming ISO and aperture stays constant.
That is the simple way to calculate.
If you want to think of stops, then each addition stop of ND will extend shutter speed by a factor of 2 (double the exposure time). So you have:
ND2 = 1 stop ND, double the exposure time
ND4 = 2 stop ND, twice double the exposure time 2^2 = 4 times
ND8 = 3 stops ND, three times double the exposure time = 2^3 = 8 times.
You will realize the calculation is already stated in the attenuation factor...