Try using the late Trevor N. Dupuy's QJMA (Quantified Judgement Model Analysis) method, described in his book Numbers, Prediction, and War. Dupuy got the Pentagon's Studies and Gaming Agency (SAGA) very angry at him, because he managed to develop a very accurate means of modeling ground combat on his own, without a big SAGA contract.
1,000,000 dead and wounded across two very large "forcible entry" amphibious invasions (Operation Olympic would've hit Kyushu; Operation Coronet would've hit the Kanto Plain on Honshu, right outside Tokyo), while fighting opponents who frankly didn't care if they died, as long as they took one gaijin with them...it's a reasonable estimate.
The Japanese plan was to use the world's largest levee en masse--basically, every able (and not-so-able) bodied person--to fight a war of attrition.