Brimstone egg laying preference

[ChrisC]

i was reading up on the species section of UKB to see how long the Brims will be feeding up when i read this "The skittle-shaped eggs are laid singly on the undersides of the youngest Buckthorn leaves at all heights on the foodplant" how set in stone is this as the 2 eggs i found were both on the upper side of quite large leaves. assuming that is that it is a brimstone egg of course.

Thanks
Chris