Find the Last Wave Transition Delay Time

Hi,

I'm trying to determine the time between the last edge transition after I set a control signal which based off a timing variable I've placed into the test bench.

I have attempted to use the cross function to achieve this with no good results.

cross(VT("/out") (0.5*VAR("vdd")) 1 "falling" t "VAR("Off")" nil)

Is there another function that can achieve this goal?

Thanks.