You may as well ask what is the perfect thickness of a memory foam pad, or what is the ideal firmness for a memory foam topper. The answer is the same – it depends on you, your mattress, the season and the temperature of your bedroom.
Why does the season and the temperature of your bedroom matter? Because memory foam is sensitive to temperature. When the room is warm, e.g. in summer, your memory foam topper pad will be softer. When the room is cold, e.g. in winter, your foam topper will be firmer.
The same point holds true for your body and your mattress. If you have a firmer mattress, you may want a softer memory foam bed topper. Same if you suffer a lot from muscle aches. If you mostly sleep on your back, you can probably make do with a thinner foam topper for your mattress.
Does all this have any relationship to the density of your memory foam mattress topper? Of course. As a rule of thumb, denser memory foam is usually firmer, and will last longer. So if you want a softer surface to sleep on, you need a memory foam mattress pad that is less dense, and you will just have to sacrifice some of the lifespan.