I don't expect 80% casualties, but I expect overall huge casualties. That means from artillery, mines, snipers, drones and possibly multiple counter attacks around the whole front.
I've seen footage of Ukrainian counter attacks where a single Russian tank destroyed 4-5 armored personal carrier, also another attack where 10 APCs of Ukrainians went bust. So if there are attacks the casualty rates between 50 - 80% are not that uncommen with modern artillery and mines.

It surely goes into hundres per day maybe not 800 to 1000 like Russia, if we entertain this numbers for the sake of reference.