Hi Jan
I use profoto equipment but not acute (pro series 8,7, and B2's) however the principals are the same for all packs from any manufacturer. Half the power to reduce the output by one stop.
The way you can get the lowest output from your 2400 is to use the B socket which goes down to 75w/s. Channel A I believe is one stop more at 150w/s.
Another factor is distance to subject of the light. For this you have to remind yourself of the inverse square law of which there is much on the web but a more visual explanation can be found here
http://photo.tutsplus.com/articles/lighting-articles/rules-for-perfect-lighting-understanding-the-inverse-square-law/If your studio is small then the amount you can use light to subject distance is obviously going to be limited.
Profoto have quite a lot of videos showing what a lot of their light modifiers will do in various situations. You will need some of these modifiers to achieve anything meaningful with your lighting and that is where I would spend your money rather than an awful lot on another pack which will give you 1 stop lower through one channel (37.5w/s). What happens when you want lots of power again?
Rather than ND filters for the camera, look at ND gels for the lighting, this can be bought in rolls or in sheets. These gels are usually expressed as 0.3, 0.6 or 0.9 but other variants exist.
.3 is 1 stop, .6 is 2 stops, and .9 is 3 stops of ND. If you buy them in roll form you can cut them to size to suit the modifier. Try to keep them off the hot glass from the modelling light.
If you invest in light modifiers you can also always rent profoto gear which does go lower in power and use your modifiers.
If you are shooting digital go out and get a decent capture software like Capture One pro which will allow you to tether to a computer and also a decent calibrated monitor. Then you can see much more visually what your light is actually doing relatively. Shoot a frame, observe, adjust a light, shoot a frame observe. Blacking out and using modelling lights on ratio will help somewhat as well.
Hope this helps
Will
www.photo.uk.com