You didn't royally mess it up, you just undercarb'd. This is a confusing point for many of those priming calculators, but they consider temperature to be fermentation temperature, or the highest temperature reached post-fermentation. Cold crashing forced some CO2 back into solution, but your beer did not have as much residual CO2 since it escaped during the 70-ish fermentation. At 2.4oz, you likely carb'd to 1.8-1.9 vol.