I think you're fine continuing with the 70 sts you have. It shouldn't affect the size too much. On the first decrease row it calls for k6, k2tog around. With 72 sts, you would do this 9 times in a round. When you get there, substitute k5, k2tog for two of those 9 repeats and you'll be right back on track with the pattern. Cute pattern, good luck!