For a prelim test the horse isn't expected to be on the bit as they are for novice horses/riders but I guess you would pick up more marks if your horse was going well in general.
You should get a mark sheet at the end of the show, with marks for each section of the test and some marks for your overall position and how the horse went for you.
If you get a copy of the test from the BHS bookshop it has a column down the right to show how many marks are awarded for each part of the test and then the marks at the end for position etc.
If it's your first test then just go in and have a go, it will be a bench mark for you to improve on next time and you can work on your lower scoring movements. Just try to be as acurate as possible with each part, things like making sure your 20m cirlce goes from one side of the school to the other and isn't more of an egg shape!!
The last prelim class I did was won by someone on 80%, but they were way out in front, I came 5th with 65 and I think the marks went down quite a bit from there.