It should be set to whatever temperature allows you to achieve optimal temps in your tank, so if you want to achieve 88 to 90 on the warm side you need to adjust until you reach those temps.

Everyone's settings are different, depends on the enclosure, substrate, thickness of substrate, t-stat, UTH brand, ambient temps etc, in other words what I set my t-stat on is irrelevant you need to find the setting that for for YOUR setup.