Hard to say,are you using Magic Tracks,workables,Fruils,or what ?
If you still have the rubber band tracks,count them,if your doing Fruils go higher,you can always pull a couple,if your gluing Magic Tracks together,go a few less,you can always add a couple.
Don't forget,how tight you glue your tracks together can make a difference,also you can adjust the idler.Depends how much sag you use.So there are a lot of factors.Just a starting point might be 90-92