A shop manual (particularly one with colour photos) will offer some spark plug diagrams / photos, to compare against your plugs, but they're not particularly useful.
As you said, claims are contradictory. The problem is, that a symptom of too lean/too rich is that it doesn't run as well as it should. That's both ways. So a too-lean symptom for one person is the same as a too-rich was for someone else. The best bet is to do one direction or the other and see if it gets better. Sometimes this can be accomplished by tricks (blowing in the vent tube to lower the gas level and lean it out (but just try to do that at full throttle!)) but more often it's going to be trial and error with part changes. This is why people pay money to dyno operators to tune the bike and do all the jet parts x throttle position x engine speed matrix testing at once. You can take the long approach and tune it yourself with ears, feel, butt-dyno and notes - lots of those! - over some time.
In order to tune your carbs you also need a better understanding of how the various circuits affect the mixture at different throttle openings.
I'd suggest starting with a read through the Mikuni VM Manual. The VM models are older, direct-action slides rather than the vacuum-operated BS34, but the same principles apply. In particular the last diagram "Functional Range Effectiveness of Tuning Parts in relation to the Throttle Valve Opening (Approximation)" illustrates which parts (mostly) affect which throttle positions. It's good that they put "(approximation)" in the title - because the diagram lies - everything seems to affect everything else, but at least it gives you an understanding to start from.
To tune your carbs, you need to either go through everything systematically, starting at the main jet and working down, or you can choose to just identify some problematic parts - particular throttle openings x engine speeds - and address those. Factory Pro has a CV carb tuning procedure for lower rpm engines that's an appropriate systematic procedure for your XS650.
