If he is already using a 10 fixed ND filter then he needs to go to a faster shutter speed to not over expose. Again, I admit I'm new and have a lot to learn but I like having the variable filter. On long exposures I set my ISO to 100 and my f-stop to whatever is suitable then I can adjust my shutter speed and ND filter until I get proper exposure. It has worked for me and I don't have to get a calculator to figure it out. I let the camera's exposure meter do the final calculation. When I have everything set right the meter shows me so.
Here's one I took at 30 sec, f18 and ISO 100. I wanted the 30 sec exposure so I adjusted the ND filter and f-stop until the camera's exposure meter showed proper exposure. I was experimenting with different exposure times and was adjusting the filter and f-stop to see what results I would get.
This is the Display I see on my Nikon D3300. The photo now shows the tic mark at "0" which is normally proper exposure. If it is on the minus side I know I'm under exposed so I have to let in more light or raise the ISO...lower f-stop or slower shutter speed. If it is on the plus side then I know I am going to be over exposed so I have to let in less light...higher f-stop or faster shutter speed. If you max out on those settings and are still over exposed then you need the ND filter to reduce the amount of light hitting the sensor.
Again, I'm just a beginner and don't claim to know exactly what I'm doing. I've read a lot, watched youtube and practiced and this is what I have taken away from all my efforts. I'm open for any advice or corrections if I've have misspoke.
I use the camera's exposure meter. ISO at 100, f stop at whatever you choose then I set the my variable ND filter to about half way and adjust the shutter speed and ND filter until I get proper exposure on the cameras exposure meter. If I want a slower shutter speed then I have to go higher with the ND filter, the faster the shutter speed the less ND filter you need. Once you go to lowest ISO and highest f-stop the only two things left to adjust are ND filter or shutter speed.
I'm new to photography myself but have done some waterfall shots. You already have the lowest ISO and you are at f22 so you will either have to stack filters to let in less light or go to a faster shutter speed if you are over exposing. The faster shutter speed makes more sense to me. When I did my water fall shots I used a variable ND filter and set my ISO to 100 and a high f-stop as you did and used my variable ND filter then dialed my shutter speed while watching the light meter to get proper exposure. As a newbie I'm open to suggestions.
The attached photo was taken with my Nikon D3300 at 3/5 sec, ISO 100, f32...I'm not sure what the ND filter was set to because I use a variable but I know I never go to the max 10 that it is capable.
I'm a beginner and I have a Canon Rebel T6 and I have a Nikon D3300 and D5100 and I must say I like the Nikons better. It could just be me or the lenses I'm using but I like the Nikons better. I feel the Nikons give me better photographs. I'm trying to use manual modes more and the Nikons are easy to use. I'm afraid my Canon camera is getting neglected. I bought the Canon brand new as a kit. I bought the 2 Nikons off of Letgo and got some really good deals. Both cameras look brand new and have low shutter counts, if you can trust the programs that give that information. I'd say from the looks of the two cameras the counts might be pretty accurate. There are good deals to be had on used cameras if you are diligent and are willing to negotiate with a stranger. I rushed in and bought the Canon and it really wasn't the camera for me. I gave more for the Canon than I did the 2 Nikons combined. You live and learn. As beginners we really don't know what is best for us and for me I think it is better to go used or rent until you know what you want. If I had it to do over I would have bought a nice used camera to learn with and then after my knowledge level was better, then I would have gotten something better if I felt I needed it. By the way I also bought a used Canon 50D off of letgo just to get the lense that came with it and got it dirt cheap but it turned out to be really nice older camera that takes very good photographs. So as you may have guessed, I went crazy buying cameras and could probably have one really nice camera for what I gave for the 4 that I now own. So don't rush in like I did. Do your research and find one nice camera to learn on and then step up later after you are more knowledgeable.