Parallax: Really just the small angle formula
The angular diameter here is
“p” – the “parallax” in arcsec.
The linear diameter is 1 AU.
d = 206265/p in AUs
d = 1/p in units of “parsecs”
1 parsec or 1pc = 206265 AU
1 pc = 3.26 light years
From the text, Horizons by Seeds