If your SSR is the 'P' version but not one with another letter after the 'P' you can forget the ZC altogether.
Not connected directly with this but worth mentioning: many of the multi-relay boards sold on Ebay that use opto-isolators are NOT actually isolated. For some bizarre reason, all the optocoupler supplies are joined together and their outputs drive transistors that also have all their emitters joined together. In other words the optocoupler does nothing useful at all. I use them because they are cheaper than building my own but I remove the PC817 and link across it's input to output pads on the PCB.
Brian.