Responsive text size?

Question

Game development

In css, you can do this in many ways, the easiest is to set the size in EM or REM, you can also use % with calc().
As I understand it, under the hood there is a calculation of the scale of the text, can I have the formula please?)

TTFConfig labelConfig;
  labelConfig.fontFilePath = "fonts/Marker Felt.ttf";
  labelConfig.fontSize = 15;
  thisSessionScoreLabel = Label::createWithTTF(
    labelConfig,
    std::to_string(thisSessionScore),
    TextHAlignment::CENTER,
    visibleSize.height * 15
  );

UDP. By far the most open answer I've found