I was one of those confused people. The solution (or a workaround) I’m currently using is described here: Wrong leading formatting in Paragraph · Issue #159 · typst/typst · GitHub :
#let leading = 1.5em
#let leading = leading - 0.75em // "Normalization"
#set block(spacing: leading)
#set par(spacing: leading)
#set par(leading: leading)
There is also a proposal to add baseline-to-baseline distance setting: Proposal: change `leading` option to `line-height` · Issue #4224 · typst/typst · GitHub. I will be very happy if it will be implemented, because this is probably the only thing that I don’t agree with Typst (the leading definition).
You should use em unit instead, because it’s font size dependent. For the default Typst 0.12.0-rc1 document, 12.6pt can be represented as 1.145em.
#context 1.145em.to-absolute()
