triac switching low voltage AC?

I need to switch a 24vac solenoid valve, using a microcontroller. I have from the surplus shop some MOC3022 opto-isolated triac drivers along with some BTA06 600v 6a "logic level" triacs.

my question is, how do I compute resistors R1 and R2 (see attached) for use with the lower voltage. R1 and R2 seem to form some sort of voltage divider, but the datasheet doesn't give any information on how they calculated those values. also, C1 should be a high voltage AC capacitor right?

secondly, is using a 600vac rated triac to switch 24vac OK or should I try to find a lower voltage model?

Ohm's law helps, find the gate trigger current in this voltage then calculate the new resistance with the new voltage and triac gate current

I would still put a bjt between the two for safety.

You might want to add a snubber network to protect the triac though.