Because search performance is an aggregate of multiple ranking factors, there's no simple answer to the question of "what score do I need in Wordmetrics"?
Ranking factors include page-speed, mobile friendliness, the number of inbound links to the page, domain authority, time spent on the page, and other metrics.
While contextual relevance is a very important factor, the degree to which you'll need a high Wordmetrics score depends somewhat on how your page already performs based on traditional ranking factors.
For established domains, increasing a post's contextual relevance can have a noticeable effect on search position and inbound traffic generation.
Scoring at least a B+ is recommended in all cases.
If the semantic index generated by Wordmetrics isn't easy to use, consider optimizing for a more focused search phrase.