Mg/kg HED (human equivalent dose) conversion factors often used  when you don't know the exact one are Mouse 11-12, rat 6-7, rabbit 4.    More or less.

E.g.,   500 mg in a 70 kg human is 7 mg/kg.   multiply times 7 (the rat HED-conversion factor) = 50 mg/kg,  about.  Thus, for a 500 gm rat the HED is 25 mg,  again,  more or less.   Unless I missed a decimal point somewhere.

one of the references of my article entitled "a critical approach to administration of low dose aspirin ..." is about the protocol of dose converting.

please refer to my profile

Nair AB, Jacob S. A simple practice guide for dose conversion between animals and human. J Basic Clin Pharma 2016;7:27-31.