Geezerhood,
I'm sorry. I guess I should have replied sooner. I was following this thread hoping for an answer myself as I have the same setup you do. It does also bug me that I cannot set my own specific zoom level, but I've become able to live with it.
The reason I say I'm sorry is that I should have thought to ask you (not what you want to do --- still don't know why others don't understand), but why. My biggest issue is not making the image bigger to a specific size, but smaller. I want to zoom out! This cannot be done via the zoom function, but can be done via the "Proof Margin" setting in Edit > Preferences > Appearance > Proof Margin, and using the slider.
I don't know if that might not be what you want, but there may be some other way to achieve what you are looking to do? Let us know why, and someone may have an idea.