Statistics: How Many Nurglings?

A full discussion of daemon troops wouldn't be complete without those naughty little nurglings. I have admittedly struggled to find a place for them in my mixed daemon forces (not so mono-Nurgle lists with Epidemius). But if they are to have a role, then is should be analogous to ripper swarms: a screening unit and a tar pit. Screening is tough to evaluate and it clearly follows the rule of thumb of the more the better. Let's have a look at the other role.

Tar-pitting.
With some luck, the nurglings will not lose a single base by the time they reach hand to hand combat with their target tar-pit unit (a squad of marines, of course). They might have taken a wound or two or one base though. So the question then becomes how many nurgling bases do I need to force a draw with the space marines in close combat?

In close combat, the marines strike first, hitting on 3+, wounding on 3+ and making the wound stick by avoiding 5+ save. For 9 marines, that means 2.67 unsaved wounds -- just about enough to take off a base of nurglings. The power fist sergeant will cause an additional 0.74 wounds, taking the total up to 3.41 wounds.

In exchange, one base of nurglings hits on 4+, wounds on 5+ and makes it stick a third of the time. With 3 attacks per base, (4 on the charge) that comes to 0.167 wounds per base (or 0.33 wounds on the charge). That means I will need at least 18 bases without charging (hahaha) and at least 9 or 10 on the charge.

So the answer to my question is that I will need to take a large (maximized) squad if I want the nurglings to do a half decent tar pitting job. Otherwise, they can simply die fast and provide a screen to my other more valuable troops!