87F on bottom is perfectly fine. I incubated half clutches this season at 87.
Just be sure to have fans blowing down to even it out more and your top temp should drop 1F by raising the bottom.
Also where is the probe positioned. Its best to have the probe at the bottom hovering mid air. Since heat rises, fans blow it back down keeping a consistent temp. Keeping probe at top the bottom will cool off a lot since all the heat is rising towards the probe.
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