Our group took it in the opposite direction:
1 weedy or poorly trained creature = 1 horde magnitude
1 average or better trained creature = 2 horde magnitude
1 very large or professionally trained creature = 3 horde magnitude
In this way, a squad of 10 imperial guardsmen is 20 horde and 10 imperial stormtroopers are 30 horde. That way small hordes can represent trained squads without things getting weird, and you can use actual imperial guard formations as combat elements quite easily.
We also assume that a casualty isn't necessarily dead (though a lot of people use that as the default), but shell-shocked, badly wounded, unconscious, terrified to inacivity, or routed are also valid explanations for horde magnitude loss.
Look at your current numbers. If someone sprays a legion heavy bolter with metalstorm shells and storm of iron at a horde of weak creatures (like a mob of rioting civilians or hormagaunts), he's going to land 6 hits of blast 2 each for 18 magnitude, plus his BS bonus for another 6 (for example) for 24 horde magnitude or 72 casualties. I know this is 40k and things are over the top, but 72 casualties from a 6-round burst!? What happens when you turn a vulcan megabolter onto a crowd (which would be admittedly gratuitous), does it wipe out the entire population of New York City?