I guess what I really want to know is 100% of what? As 100% is a relative size and not an absolute size like a pixel value.
Topic summary
Base font size in Shopify’s Dawn theme is set on the html element: font-size: calc(var(–font-body-scale) * 62.5%). 62.5% of the browser default (16px) equals 10px, so at a body scale of 100% (i.e., 1) the root font size is 10px. rem units reference this root size.
Theme controls for the scale are in config/settings_data.json: heading scale and body scale (e.g., body scale 100% = 1 → 10px; 130% = 1.3 → 13px). The “100%” refers to the theme’s body scale variable, which multiplies the fixed 62.5% base.
Rationale: a 10px base is used to simplify rem/em calculations. The original poster confirmed this understanding and was satisfied.
Status: resolved; no further action identified.