Hi,
I averaged 75 lines on the dots test but as has been suggested on here, 60 on average should be good enough. However, the average is also dependant on the class, so if everyone performs well then 60 might not be good enough. I believe though, historically - aim for 60 with no errors or omissions.
As for the mechanical test, unfortunately (fortunately) Southern no longer require such a test, I had heard from many on here that a lot of TOC's no longer use it. But if you think yours will then definitely prepare for it.
When practising the dots test, take lots of deep breaths as this helps keep your heart rate down and stops you panicking which can lead to errors.
Otherwise, just try to keep calm and remember to take your time to be accurate more than anything else.
Good luck for tomorrow - you'll be alright!