Question General guideline to placing reflection probes?

Is there a guideline on how to place reflection probe I can read somewhere?
Since HDRP relies heavily on reflection probe fallback to get passable GI, it would be good to read how expert approach on where to place them. What are the criteria? Where to start? What to look for?
Right now, I admittedly just go for center of room first then adjust/subdivide from there. But things get blurry for an outdoor scene.

 

Center of the room is a pretty good starting point and turning on box projection for those interior probes should get you pretty decent results in small - medium sized room. You can go further from there by adding extra probes focusing on areas with very smooth materials where the inaccuracies of reflection probes will be noticed more easily.

As for exterior scenes the blurrier reflections are a side effect of the reflection probes being applied over a larger area without increasing the resolution or the number of probes to compensate. Having a large of high resolution probes isn't generally a great idea because of the amount of storage space they take up. An alternative that you could use is having a realtime reflection probe follow the player around. By default this would obviously be rather expensive but you can do things like enable time slicing, lower the resolution of the probe, and adjust the custom frame settings of the probe to find a balance between performance and quality.

There are some great examples of what SSGI + realtime reflection probe look like in this thread
https://forum.unity.com/threads/ssg...ation-noise-denoiser-and-performance.1211346/