Normalize the tempo first
Compare songs at their musically perceived tempo. A 70 BPM hip-hop track may relate naturally to a 140 BPM drum-and-bass count. Test half- and double-time interpretations before rejecting a pairing.
For transparent full-length overlays, smaller percentage changes usually sound better. For transitions, a larger gap can become part of the effect.
Translate keys into one system
Traditional key names and Camelot codes describe the same tonal centers in different ways. DJs use Camelot notation because neighboring relationships are quick to scan.
Same-code blends are stable. Moving one number clockwise or counterclockwise with the same letter is usually smooth. Switching between A and B on the same number moves between relative minor and major.
Use numbers to choose what to audition
A database can reduce a thousand possibilities to twenty. It cannot hear vocal cadence, sound design or arrangement density. Use metadata to build a shortlist, then audition short sections.
When a promising idea fails, diagnose the failure: harmony, phrase alignment, conflicting bass or simply too much information.