Hi,
A quirk of sclang - if you use pipe syntax, you have to surround an expression with parenthases, so |modFreq = (440/60)|
See Functions | SuperCollider 3.12.2 Help
Edit: OH but actually this might still not work in a synthdef function (not at computer now, sorry) – in any case check out NamedControls for fewer surprises overall: NamedControl: a better way to write SynthDef arguments