Thanks Oginme,
I've re-visited the equipment profile, and now with the same recipe, the amount of sparge water computed is 39.11 litres, resulting in a boil volume of 58.0 litres, compared with 63.65 litres yesterday.
This agrees closely now with yesterday's actuals, so I'll see how it goes next brew,and tweak it from there if necessary.
Thanks again,
Cheers