We propose a new model calculation using the reversal probability of grain magnetization for heat-assisted magnetic recording (HAMR). Our new model calculation can obtain the bit error rate as a function of the writing field Hw for a given anisotropy constant ratio Ku / Kbulk, which is a new parameter that we introduced. The physical implication of the recording time window proposed in the micromagnetic calculation is discussed using the new model calculation. Although the recording time window is a good guideline for write-error and erasure-after-write (EAW), EAW cannot be determined solely by the recording time window. The influence of EAW is accurately examined. The allowable range of Hw and Ku / Kbulk is also provided for various Curie temperatures Tc. Tc and the heat-transfer thermal gradient are important parameters for reducing Ku / Kbulk.