An additional constraint that makes it hard to apply a uniform spacing is that stops should optimally be placed at intersections with traffic signals or crosswalks (as is currently usually the case). There are cases where it might seem logical to replace two existing stops with a new one in between, but if the two existing stops are at traffic signals and the one in between is not, then the result could actually be more stopping (i.e. at the two red lights and at the stop in between).
I wasn't suggesting to do it without thinking. "Optimal spacing" isn't a hard number, it's an average that you aim for.
That said, there are clearly a number of locations where traffic signals and crosswalks are located there BECAUSE that's where the streetcar stop is. When this is the case it could be worth relocating the lights and crosswalks if that could permit better spacing.