My simulated engine out testing has shown 550-600fpm at 100kias with 3/4 fuel and just me in the airplane.

I had a real engine out up at 10k a while ago.  Descent rate at 100kias up at 10k was right around 1000fpm.  SOME of that could be attributed to the higher TAS. SOME of it could be attributed to the higher weight. SOME of it could be attributed to me flying slightly faster than 100kias. The rest, I assume, is the difference between idle thrust and a zero combustion excursion.

Keith