We could leave the must but change the if to "if a percentage based score is known"
Also, "not 'raw'" is wrong, it's valid to include scaled _and_ raw, the point is if the % is known, include it in score.scaled.

This isn't a particularly testable if, based on which it might be better to go with "SHOULD" as you say, but perhaps with the final text being:

SHOULD include 'scaled' if a logical % based score is known.