So relatively early in the boil before the gravity creeps past 1.060, or perhaps 1.050 even? I like the idea of a fresh wort starter, seems like a better representative of what the yeast will be exposed to anyway upon inoculation of the remainder of the batch.I reckom Mr malty overestimates but even so, 1060 is the uppermost limit of the wyeast recommendation. 1070 is above that and by enough to make me want to grow some more yeast.
Anything above about 1050 ish, I make starters (often make them below as well but only because I often recoup some of my worty trub by allowing separation overnight).
To answer the first question - I make all my starters from the same wort that will be fermented. I rarely dilute.