Its always difficult to be definitive with temperature recommendations as everyone tends to respond differently to cold and also the thermal insulation offered by different drysuits varies from a neoprene suit to a membrane or trilaminate.
I general, under a trilaminate suit I would recommend the Arctic for temperatures from the mid to upper 50s to the upper 60s. In temperatures lower than 55 or so, I would strongly advocate layering the Arctic with a xerotherm base layer or a similar thermal base layer.
Under a neoprene suit, depending on the thickness of the neoprene we recommend the Arctic for cooler temperatures down to the upper 40s when worn on its own. In the lower and mid 40s we would recommend layering the Arctic with a xerotherm base layer, or similar base layer.
I hope you understand how difficult it is to be prescriptive with this, and people's individual experiences vary wildly.
Under a trilaminate or membrane suit in temperatures of the upper 40s and cooler, down to freezing, we reccommend the SubX undersuit as the principal thermal protection.
I hope this helps.
Jim