There is an analysis option "stb" (stability) which with the iprobe
"widget" does it all; you can plot traditional Bode plots or a margin
summary.
Otherwise it's make the gain expression, use the "cross"
function to find the zero gain, use that value as the argument
for value() on the phase expression.
Otherwise it's make the gain expression, use the "cross"
function to find the zero gain, use that value as the argument
for value() on the phase expression.