Hi Vermis,
I use the WM bases in threes as that's how I had them grouped for Warmaster. So each group of 3 is a single unit in DR.
Each foot unit can take a total of 12 hits, so whenever I take 4 I remove a base, recording extras with the mini dice.
mounted troops is 6 hits per unit, so I remove a base for every two hits taken.
I suppose you could use either a single WM base or two, or even, six or twelve but as mine are painted in groups of three (matching shields/cloaks etc.) I just kept them like that.
In my 28mm Lion Rampant army I've got units of 12 foot and 6 mounted, so just take off a model per casualty.
Does that all make sense?