[Syrphidae] Melanostoma scalare or mellinum?

[Fraser, SEM]

I have recently started to identify hoverflies that I  have collected in
Malaise traps as part of my PhD at the University of York. I am getting
a lot of Melanostoma but am having difficulty separating males into
scalare or mellinum. Using the Stubbs and Falk key I have trouble
deciding whether Tergites 2 and 3 are twice as long as wide. In many
cases tergite 2 seems double the width in length but tergite three is
probably only one and a half time as long. This still seems elongate
when I compare them the specimens in which the tergites are more square
than elongate. Is there any way of overcoming this issue?
Thanks
Sally Fraser