A Neutral Density filter block light getting into the lens.
IF you are using an 'auto' exposure mode on the camera, that measures the light TTL or Through-Taking-Lens... it should compensate for the 'filter factor' because the meter only sees the light that gets through the lens, AND the filter, and bases settings on that meter reading.
So, if metering full Auto, Aperture-Priority or similar you shouldn't need to do anything. If shooting in Manual mode.... where do you get your meter reading from to make your own shutter/aperture/ISO settings from? Most likely the Hi/Lo indicator in the vier-finder, from TTL meter.... same deal applies.... that sees light after the filter, it should compensate for the filter.
The 'Niggle' is if you are using a heavy ND or big-stoppa filter..... apart from the cliche of over-milked waterfalls and waves, which is a BIG niggle in my book, but still.... the niggle, is that with the filter in place.... you probably cant see bog all through the lens in the view-finder....... this begs removing the filter to compose shot, then replacing to expose.....
- If in an auto or semi auto mode, camera should re-meter after you replace filter, and recalculate exposure settings; you aught not have to make any correction or compensation.
- In manual.... as before; if you use the Hi/Lo TTL meter in the view-finder, to make settings, after composition, and after adding filter; that should be good.
The issue is if you meter before adding filter, and then make settings, when you will have to add in the filter factor, and if its say a 3-Stop filter, add three stops of shutter steed, or three stops of aperture or three stops of ISO, or permutations of.
A-N-D filter-factors provided by filter makers aren't always all that reliable, and you may have to adjust exposure a stop or tow either way depending on results.
There's also the small niggle in semi auto modes, particularly shutter priority, that there isn't enough f-stops on the aperture for the metering system to open up enough stops for the shutter speed you set... milking waves or waterfalls, this is unlikely an issue as you will probably use aperture priority, but worth noting; it can hit the buffers and not give settings to balance exposure metering.
BUT using 'Coupled' Through-Taking-Lens metering, as built in to most modern cameras... slapping filter in-front of lens, metering should work as intended with no other intervention from you... a LONG as you let the camera meter through the filter.
If you are using a hand-held meter, and taking an incident or reflected light reading, then the filter factor is more important, and again, you have to add the filter factor into your computations to derive aperture/shutter/ISO settings... and the filter factor quoted by the maker may not be all that accurate, and on heavier big-stoppa filters, and more cheaper ones, be even more inaccurate; on a cheap heavy big-stopps with quoted filter factor of say 10-stops, you might get under-exposed images, and have to up exposure by as many as 3-stops to get an acceptable exposure...... so some trial and error may still be required.
It all depends on how you are metering....
A-N-D.. having mentioned milking water-falls..... worth mentioning that if your objective is actually very very long exposure effects..... metering is likely to go a bit to pot any-way.....
Metering measures the intensity if light at the instant of measurement. If you have a water-fall, you have a scene with highlights catching the drops of water, that are moving down the frame. In the instant of metering, or short exposure, they don't move very far.. give it a long exposure, they do... now you have the same high-lights being recorded over much larger portion of the frame, during that long exposure time..... depending on how many high-lights and how far they move, and how long your long exposure is, the accuracy of a instantaneous exposure value can vary from not far off to hugely off.... THIS is one of the reasons that milking waterfalls was set as an academic exercise, NOT in effect, or artistic interpretation, but where the mechanics of the machine fall down, and experience and judgement come into play to dial in compensation, and to judge the most pleasant exposure A-N-D effect, and NOT be reliant on the maths and the meters.... and that is the lesson now so often NOT learned by the exercise, trying to do it all by the books and the numbers in the digital world. Which may be worth taking note of.