Staff size on Oboe-Piano score

[KlausWH]

I am engraving an Oboe + Piano Score. It will be A4, and the Piano and Oboe system on the full score must share the same staff size.

If I want to fir 4 systems on one page, I can set the staff size to 6.2mm at most.

I'm not sure what the standard is for this. I find 6.2mm a little small, and wish I could have 7mm or at least 6.5mm.

At 7mm I can only fit three systems per page, which leaves these ugly, large gaps between systems.

What would others do in this scenario?

[OCTO]

A4 is already difficult for scores and parts.
If it is "a must" then the recipient must suffer a bit: use 6.2 mm as your last offer.