Besides the great advice from John W, I have found here in California that a new model called “chill portion” is more accurate in some crops than chill hours. This is especially so for pistachios, but it may also be for plums too if they are high chill hour varieties. Basically, chill portion looks at only a narrow window of chill temperatures, from 35F to 55F. Above or below that, you subtract the hours from those in the chill window. Because we are having some significant warm weather events in winter and early spring here, we have found many times that even though we hit chill hours (say, 900 hours), because we had lots of warm days with no chill, the effective chill was much lower and there was very poor crop set. The University of California has tracked this more than a decade, and have a “dynamic model” now. You might check it out:https://ccag-eh.ucanr.edu/Chill_Portions_and_the_Dynamic_Model/