I've been reading some of the recent posts with interest, but what's
still not clear to me is how much starter peopler are using in their
dough.

If I have a walnut-size stiff-dough starter that has not been in the
fridge recently (that is, I halve, refresh, and keep on countertop),
should I ramp up the starter quantity to 20% or so of the total dough
before making the dough, or just put in a walnut's worth?

While we're on the topic, just what do you think is a good percentage?

I'm also interested in your views on whether to double, quadruple,
octuple, or what.

My start is working fine, as far as the yeast goes, but it doesn't have
that classic taste anymore. I don't need really sour bread, I like
mild, but that characteristic taste is nice to have. I think I will try
a no-fridge experiment for a couple of weeks and see if that makes a
difference in the health of my starter.