I assume there's got to be a ground universe somewhere in the chain.

I'm not saying you're wrong to think this is likely, but I don't think this is as necessary a condition as some people are taking it to be. So long as each simulation is simulated from somewhere, there's no reason why it can't be the case that every simulator is also simulated. I can think of no reason why the universe would be like this, but I can also think of no reason why it can't be that way.

The chance that we are in an ancestor simulation is in the neighborhood of epsilon.

I also put the chance for an ancestor simulation in that neighborhood, I just don't give a high probability that my best guesses on the topic are actually sound and that skews things a little. ( as in 'There might be reasons why a simulation-capable society would run more ancestral simulations than other kinds of simulations even if I haven't thought of those reasons yet')

All the arguments which imply that we might be living in a simulation would imply that the universe which is simulating us is also a simulation. I assume there's got to be a ground universe somewhere in the chain.

Yup, there must be a base universe but it is possible that the inhabitants of the base universe have also done their math and also reason that the chances are higher for them to be in a simulation (even though they are actually the only ones(not actually sure about the 'only' bit) that are not in a simulation).

An extreme Singularity doesn't seem necessary to get a lot of simulations.

Not an extreme one, no. Still, you need to have passed a certain level to be able to run those simulations and you can progress your civilization a lot more just from information that you gather from such simulations which in turn would logically lead to an 'extreme' singularity. (exponential growth and all).