Duncan - I spend virtually all my time in diagrams but the interesting thing is that I'd never hit the problem that you describe - indeed I looked through 3 top level menus (Diagram, then View, then Edit) before I found that "Find in Diagram" is a command on the 3rd of those, so it's clearly not something I use much!
So, with my apologies if you don't want to, or can't, work this way, may I suggest that you don't try to confine yourself to one diagram, simply because it's so easy to generate a new one to appear as well as the old one.
The way I work is something like this.
Firstly you need to make sure that you have the Navigation Bar switched on (View / Navigation bar) because if you have multiple diagrams showing, that's the fast way to go between them.
Secondly, I have my Individuals tab set to show the first spouse against each individual in the Individual Records List. That gives me a fighting chance of telling one John Smith from the next.
Now, suppose that I am working in a diagram showing my ancestor John Purdy and want to go a part of a diagram showing the Salters who are descended from him. I *could* open the next generations further down and use Edit / Find In Diagram to find Thomas Salter (I happen to remember he's the one who married a Purdy). But this has a few limitations - firstly Edit / Find In Diagram only finds those people actually visible on the diagram. If my expansion button has closed his branch, or I'm not showing sufficient generations (same thing really), then Edit / Find In Diagram won't find him. Also, as soon as I try to get lots of people in one diagram, I can't read the boxes.
So, what I do is go from John Purdy's diagram to the Navigation Bar, in order to see the Individual Records list. Use the Filter facility at the top of that tab to filter down to last name Salter, first name Thomas. In that list, because I have it showing the first spouse, I can see the "Salter, Thomas" who married Sarah Purdy. Select him and choose your favourite diagram icon. And there you have two diagrams, open at once, showing both John Purdy and Thomas Salter. If I need to see the connection, I can select Thomas' wife, Sarah Purdy, and bring up an Ancestor Diagram that goes back to John Purdy - at this point I have 3 diagrams open at once and I can easily use the Navigation Bar to swap between them.
As I said, if you have some reason that you can't work like that, fine, but otherwise I'd really suggest that you give multiple diagrams a go. Particularly if, like me, you only have a smallish sized screen.