Your diff should just be tight enough so it doesn't make what I call a "Barking sound" under acceleration. Don't over tighten it or you will be rebuilding every two mins.
You need to set the slipper so it slips for about half a metre. For different track conditions you'll have to set it accordingly.
You can hold the back wheels and do it that way but it's better to take the nut spinner to the track and keep giving it little blips until you get it just right.(if that makes sense
)
Phill