The transition from sounding out unfamiliar words to effortlessly reading connected text does not occur all at once or at the same rate for students. The purpose of this study was to explore the accuracy of three decision rules (data point, median, and trend line) applied to progress monitoring outcomes of alphabetic principle (nonsense word fluency [NWF]) and oral reading rate (curriculum-based measurement of reading [CBM-R]). Outcomes from a sample of U.S. students receiving Tier-2 supports in oral reading and decoding were analyzed to generate model parameters. Scores were simulated for NWF and CBM-R, and decision rules were applied to schedules where one observation was collected per week. The trend-line rule was viable with NWF after 7 weeks and 9 to 10 weeks with CBM-R. Differences in base rates of non-proficiency between measures call into question the utility of NWF to capture student improvement in alphabetic principle as they encounter increasingly complex word types.