You are correct. Photoshop will not touch your scratch disk if it has enough memory.
Optimal memory settings are usually allocating around 70% of total ram to Photoshop.
Photoshop Memory Usage setting Using 70% is the smart move until and
unless tests show that more is faster.
The operating system version, Photoshop version, other programs, etc,
all influence whether too high a setting improves performance, or
instead causes virtual memory paging and/or degrades the ability of
the system to have enough spare memory for drive caching. It is a
performance see-saw effect under some conditions.
-- Mac Performance Guide
Also to note is that Photoshop does not pre-allocate the memory. It only uses it if it needs it.
Enough is enough
Remember too that enough is enough— Photoshop will use what it needs,
up to the limit set with this preference.
Using a setting of 90% instead of 70% is meaningless when Photoshop
really only needs and uses 40%. Photoshop will use what it needs up to
the limit; it does not pre-allocate the memory, it’s simply a future
limit when Photoshop is launched.