"Are there any guidelines regarding what ratio of acrylic paint from the tube to medium I should use?" -- Diane
When it comes to thinning acrylics, the only 'rule' is to not mix acrylic paint with more than 50 per cent water. Any more than this and it may loose its adhesive qualities and peel off at some stage, or lift when you paint over it. You can mix in as much acrylic medium (glazing, texture paste, etc) as you like because it's got the acrylic resin in it that acts as the 'glue' that makes the paint 'stick'. (Golden describe their mediums as 'colorless paint'!)
If you're new to glazing, I recommend taking a small container and mixing some paint with 50 percent water (judge it by volume), then mixing the two together thoroughly, to get a feel for just how much water this is.