Hi,

I’ve worked on a few kitchen enclosure layouts and had similar questions when planning cabinet spacing. For fridge installations, the horizontal rail depth usually depends on the overall enclosure design and how much clearance you need for ventilation and door movement, so it’s good that you’re checking different size ranges.
When it comes to the cabinet above the fridge, it’s definitely important to factor in the horizontal rail height (including that 3/4″ thickness) so everything aligns properly and you don’t lose internal storage space unexpectedly. Small measurements like that can really affect the final fit.
On the side panels, your understanding sounds mostly correct—allowing a slight adjustment (like that extra 5/8″ depending on material thickness and framing style) is often done to ensure a clean flush finish.
While working on a similar project, I also came across some Home upgrade discussions and tools that helped me think more carefully about layout precision, especially when dealing with tight kitchen spaces.
One unrelated thing I recently explored during home setup planning was outdoor and entertainment furniture like outdoor television stands, which also require similar attention to stability and measurement accuracy when placing heavier equipment.
Hope this helps a bit—curious to hear how your build turns out.

12″ is about standard for the depth of the rail.

Personally, I would make it full depth if I had the budget. That is mainly because my eye level is near the top of the refrigerator, around 72″, so I am looking straight into the void above the refrigerator. Having it full depth prevents me from seeing the exposed underside of the cabinet above, since I can literally see all the way back into the opening and to the wall.