On what temperature should I leave my sauce to cook?
Once it's reached a boil, then drop it to the lowest your burner will go.

With the lid on, the idea is to have just noticeable movement, with as few bubbles as you can get. 

Many burners won't go that low; but a tomato sauce should be OK at a low simmer for an hour, so long as you stir it every 10 mins &amp; make sure it doesn't start to stick. If you take the lid off the temperature will drop, but the sauce will reduce quicker.

Alternatively, get one of these simmer rings - a few $£€ on eBay from China. That will let you drop the temperature to lower than bubbling, almost slow-cooker style.
I use one on gas. I don't know what else it's compatible with.