Assuming I have slighlty too high velocity vector field values in my source and want it to affect particles weaker, how can I do that?
If I create Stoke, load my source, load my particles and then add Magma to the stoke with input channel velocity > divide by 10 > output velocity would that work?
It does not seem so…
There is a Velocity Scale property per Field.
Select a field, change the Scale from 1.0 to 0.5 and you will get half the influence. Similar to how FumeFX Follow works in PFlow.
The scale factor will be applied to the values driving your particles, regardless of their source (particles, space warps, Fume, Ember etc.).
The Scale value is also animatable per object - see the first demo in the videos section!
thinkboxsoftware.com/stoke-mx-tutorials
Getting through it now 
Thanks.