The formula for the calculation is given here: http://www.salisburywatermeadows.org.uk/waterframe-water-vole-information.htm
("The predictive equation to estimate water vole population density can be applied using the latrine numbers. This is y = 1.48+0.638x, where y = water voles and x =latrines"). However, its accuracy would depend on being able to locate all the latrines in a given stretch, and sometimes they can be extremely hard to find. This is the first and only one I've seen at White Lion Meadow car park. Again, at least one of the droppings is pointy at the end, despite what the text books say.
It's the females who make the latrines, depositing about six up and down the range of their territory and, of those, the ones marking the boundaries being trampled.