Yeah I use Argus Monitor too, good software.
In terms of when they should start ramping up though, that will need some experimentation to determine, it depends heavily on your individual setup.
Put both your GPU and CPU under 100% load since you need to plan for worst-case scenario, not just average load. Monitor the water temperature and CPU and GPU temps with whatever fan speed you have now - see if they eventually level off before your GPU or CPU reach a temperature higher than you are happy with. It could take a long while for the temperature to stop climbing, a 5-10 minute test won't be enough here.
If they get too warm and the water temperature is still climbing, make a note of what temperature the water was when that point was reached. In my case for example with a Corsair H150i AIO (yeah I know, AIO, not "proper" water cooling, but the same principle applies) if the coolant reaches 40c, then my CPU starts to hit over 80c, which is more than I'm happy with even though it's not going to actually throttle.
Make that temperature your target maximum, then work out what fan speed you need to not exceed that under 100% load - experiment with setting the fans at different static speeds (e.g. lock all fans at 50%, or 70%, or whatever). That should then be the maximum % you need to set on your fan curve, for that water temperature. Again in my specific case, that means the CPU fans at 90% at 40c - you might well need lower depending on number and size of radiators etc...
If on the other hand your water temperature already stops increasing before your GPU or CPU get warmer than you want, you can afford to reduce your fan speed and have things quieter - experiment with lowering the maximum until you find a suitable limit.
Once you know your target maximum fan speed, work out a suitable minimum too. This time leave both the GPU and CPU idle, and try locking the fans at various speeds until you reach a point at which they are inaudible to you - in my case I can't hear them from where I sit when they are at 35% speed, which is about 500 RPM. Locked at that speed at idle, the CPU coolant in my setup sits at 35c.
See what water temperature you get with the fans locked at that virtually silent speed, and make that the minimum for your fan curve.
Finally, just define a smooth ramp between the minimum and maximums you determine - if you have it fairly linear the fans should increase (or decrease) nice and smoothly without big jumps in speed which will be noticeable.
As an example, here's my fan curve for controlling the speed of my case fans depending on my GPU temperature:
I found that my GPU won't go over 45c when just using standard desktop apps (not playing video or games) even with the case fans stopped entirely, so I turn them off completely below that. Then just 20% at 45c for a nice quiet start, rising smoothly to 40% at 75c - my experimentation showed they don't need to be higher than that to keep the temperature under control.
I set a fairly steep rise after 75c just in case things start overheating somehow, but in practice I never reach that point under usual gaming load - the temperature stops climbing before it reaches that point.
