I see I should put up a link to ACT and SAT score comparisons to answer another FAQ.
SAT ACT concordance - Google Search
The short answer is that colleges decide, college by college, how they relate ACT scores to SAT scores, either basing their comparison table on data from their own previous applicants or using the tables published by College Board or ACT. Every major college has a sense of what a high SAT score looks like, and what a high ACT score looks like, and the colleges say they consider an applicant's highest scores.