(1) Subject to the other provisions of these rules, all relevant evidence is admissible which, in the opinion of the hearing officer or board, is the kind of evidence on which reasonably prudent persons are accustomed to rely in the conduct of their business, or is the best evidence reasonably obtainable, having due regard for its necessity, availability, and trustworthiness.

(2) If not inconsistent with subsection (1) of this section, the hearing officer shall refer to the Washington rules of evidence as guidelines for evidentiary rulings.