1/4" drop at the lowest point is normal. I used to ride a B17 with 4mm thick leather. This saddle sagged badly (1/3"-1/2") after only 800 miles.
Switched to an Imperial Narrow with 5mm thick leather. The skirts were laced tight. +3600 miles with only 1/4" gap at the lowest point. Lacing the skirts should drastically reduce sagging.
What is the nominal thickness of that Brooks Pro?