Here is a nice little guide to building an ansmann diff.
http://mrmagoo-rc.blogspot.co.uk/201...ball-diff.html
Ideally you want the diff to be nice and free when you rotate one of the wheels, with the opposite wheel turning in the opposite direction. You also want it to not slip at all during use. So if you lock the slipper up hold both of the wheels you shouldn't be able to turn the spur gear.
Once you have got the diff set correctly you want to set the slipper up. This should be tight enough to not slip too much but not completely tight so that the car pulls wheelies everywhere. Just set it so slips a little bit on intial accelleration but then stops slipping as the car is accellerating.
As you have a madrat there is a Gear differential available. These are easier to set-up and maintain.
http://www.eurorc.com/product/2254/t...ff-set-for-2wd
I believe they availiable via SMD to.