On hot, dry days in the States I think it is possible to reach evaporation rates as high as 0.25" per day.
One test would be for you to set out an evaporation pan in some spot near the lake. (Probably need to put a little weld-wire over it to keep the wildlife out.) Check your actual evaporation rate. If you place it in the shade, it will probably be an underestimate. If you place it in the sun, a little overestimate. (A shallow pan will be subject to more solar heating and subsequent evaporation than your pond.)
I would use a five gallon bucket and monitor for a week. Personally I would not be concerned about 0.25" per day.
