SY Persei is a semiregular variable (SRa subclass) of long period, over 450 days. It undergoes reasonably regular pulsations, varying between 8.5 and 13.5, but the light curve varies enough from cycle to cycle to put it in the semiregular (rather than Mira) class of variables. It's also a carbon star, and you'll note that the light curves of many carbon stars show significant scatter because of their red colors. Try making observations with a "quick glance" -- that will help lower the scatter. Otherwise, make sure all of your observations are made the same way, so that we can more easily estimate your zero point differences with other observers. If you're new to observing, take a look at our Visual Observing Manual to learn more tips and tricks to making visual estimates.