There SHOULD be a temp gradient of about 10 degrees.... that being said, most young BPs (up to about a year to a year and a half) can be kept in a tad smaller gradient. A hot BP is a hungry BP.... so the warmer you keep your ambients and the cool side, the faster he'll digest.

If you can get the hot side up to 92-94, then the 85 cool side temp shouldn't be much of a problem.

Hope this helps...