Hi MichelleB.
Welcome to our Mattress Forum. 
It looks like testing in person helped answer your question. 1" is what we would recommend for a flat sleeping profile (like stomach sleeping).
Side sleeping is the most common sleeping position and because it has a more “curvy” profile than the other two positions, pressure relief becomes an important part of the choice here so you don’t end up sleeping on your more pointy or bony parts or to use a technical term … your bony prominences. These are of course your hips and your shoulders. There are deeper gaps to “fill in” to spread your weight out on the mattress when you are sleeping on your side so side sleepers need a deeper cradle to relieve pressure so the gaps are filled in and help spread the weight over the mattress. In other words they need to “sink IN” to the comfort layers more. This means that the comfort layers generally need to be a little softer and/or thicker than the other 2 positions. In general a good starting point for a good comfort layer for a side sleeper is 3" and then depending on weight, body shape, preferences, and the firmness of the support layers, to go up or down from there. Most side sleepers will fall in the range of from 2" -4". Without an appropriate comfort layer, a side sleeper will end up with pressure points during the course of the night and could have symptoms of numbness, soreness, localized redness, or end up tossing and turning all night as your body tries to relieve the pressure.
I hope this helps!
NikkiTMU