Net Endurance Gain...?

I understand that this would differ depending on your build...

...but do you shoot for any "sweet spot" end/sec when it comes to net endurance gain?

I try to have a value very close to my attack chain's end/sec

Doesn't have to be equal to or greater than, but that helps. A very slow leak is alright--you only need enough end to get you through most battles.

If you want to get technical about it, you can figure out how much end/sec you spend on your attacks and other powers, add it to the cost of your toggles, and compare to the amount of endurance you recover. If your consumption is much higher than your recovery (as a rule of thumb I use "1 end/sec or more difference"), you'll run dry pretty easily, although some builds can handle it with things like Conserve Power or Power Sink. If your recovery exceeds your consumption, you can sustain your attacks forever.

But usually you don't need to be sustainable forever, just as long as a fight lasts. When I'm not building specifically to be indefinitely sustainable I usually just eyeball it - are powers slotted with appropriate amounts of endredux? Proper slotting in Stamina, and possibly other relevant powers? Numina and Miracle uniques, Performance Shifter proc? If yes to all, probably good to go.

I always aim for 2 eps gain after toggles on all my toons.

You could add up the EPS of your attack chain and clicks then add those to your toggles and know your total end burn. I have found that this lets me run 3-6 minutes non stop for most of my toons and aside from AV/GM solo'ing it is a rare fight that takes that long.

As a general starting point in Mids I'm looking for 2 to 2.5 end/sec for sets with no significant ability to recovery or gain endurance. No Consume, Energize, Power Sink or Heat Loss for instance. For endgame I'm looking for something a bit higher, closer to 3.0 end/sec generally.
Unless it's in a set team situation where I know power "x" is being used/stacked etc., anything under 1.0 end/sec in Mids would make me very nervous. And under 1.5% I know that my blue bar is always going to be a factor. I've run both an AR/Dev and a Night Widow to 5O without Fitness (both before recent Inherent Fitness changes) and watching my blue bar was a constant as much as keeping an eye on my health.