Help:LACE PATTERN SIZE INCREASE

I’m starting a shawl lace pattern (multiples 12+6). Cast is 78 sts for a finished size of 18in. I want to make it at least 24in. How many more sts do I need to cast on?
LOL It sounds like a math problem on a school worksheet:teehee:
Thanks,

102

78-6=72
72/18=4
4x24 =96
96+6 =102

Wow! Thank you so much. I would have never been able to figure that out.