It needs to be as far forward as is sensible. Having a 4x4 with a side opening door I need a lot of space, but work it out for your rig. The axle needs to move forward or back the same distance you move the winch post/snubber.
Dont forget to check the rollers nearest the stern havent rolled out the back of the boat, they're a bugger to get back under the keel/transome.
The resulting nose weight should be within the specification of your vehicles. Usually 50Kg for a car or 75Kg for a 4x4. I find a weight of about 30-40 Kg is excellent for stability but hard work hitching on and off.
It really is something that has to experimented with. Too light and the trailler can tip up with someone in and could be difficult to tow due to snaking. Too heavy and it makes manouvering off the car hard work.
Sensible advice is to start heavy and work towards lightening the nose until you are happy.
Searider - The Best 5.4 x Far