Also, isn't your PID telling you exactly when your element is firing? It has a light, it blinks...what are you firing the SSR from, control-wise? My Auber PID has a nice light that coincide's with the SSR firing.
But yeah voltage measuring through an SSR is whack jack. Fully closed I get full voltage. Don't bother. Use a "does my element turn on and heat properly" measure, instead.
But really I wouldn't bother putting in a light to tell you when your SSR is firing or not, if you need that then mount the SSR in such a way that it's visible.
It has a plastic case on it and a light on it for this reason, no?