To be honest this whole evaporation lark has been baffling me for months ???. Even to the point when after the level dropped by six inches overnight I was considering re-lining the whole pond again as I thought I had a leak!!! I suspect that 6 inches probably equates to in excess of 100 litres in real terms for the surface area of my pond. I refilled then and it has hardly dropped since apart from perhaps the odd centimetre here and there in particularly warm/windy spells. I'm certain that the wind plays an equally signifcant part in water loss as temp does. In fact come to think of it, my pond seems to lose more water when it's windy than it does when it's hot and still.
I've also heard Margaret mention a few times that if the pond is heavily planted with marginals then they can use up a lot of the water in particularly in warm conditions as well.
lol - I've not even answered your question!!! In short I guess it depends on how big, how windy, how warm, how much planting and how big the surface area. I've sort of got to know mine and will probably only worry only if it drops below anything it has done previously or if it drops when the prevailing weather conditions would suggest it shouldn't.
