Photoshop :: How To Determine Scratch Disc Size

once i was reading an adobe pdf "How to get better performance in photoshop cs5" - that was in 2009 or 2010, and may be outdated, but there was a calculation method to determine the size of a scratch disc. (similar calculation see below, if i can remember right)
 
i am asking myself, how can i determine the correct size of an external SSD-scratch disc, only used by photoshop (completely empty):should i buy a 128GB or 256GB or 512GB SSD which is only reserved for photoshop?
 
basic question 1 : i guess i should avoid to set the internal SSD as photoshop scratch disc, as it slows down everything?

basic question 2 : in sense of maximum performance: better buy an external USB3.0 or thunderbolt SSD? will photoshop really use the extra thunderbolt speed when swapping data?
 
secondary question:
 
can i calculate the size regarding my daily working habits?
 
i am mainly working like this:
 
- with my imac 27" late 2013 with 32GB RAM and 256 GB internal pci-e SSD (800 MB/sec), which will stay always half empty for performance reasons.
- OSX 10.8 mountain lion and 10.9 mavericks soon
- photoshop cs5, cs6 and cc (always without extended)
- 8bit and 16bit mode
- only RGB
- with latest phocus/Hasselblad and canon RAW Files which produce a basic .psb document at ...
- 10.000 x 7000 px at 300dpi

[code]....
 
i wonder how to calculate ?
 
for example:

10.000 x 7000 px at 300dpi needs for one image layer at 16bit: 2GB RAM in photoshop cs6 or cc (just as a number), this may be wrong so lets take 2GB RAM and multiply with 10 image layers in my .psb file (16bit) = 20 GB RAM, and multiply with 20 adjustment layers (guess they need less ram, for one lets say 500MB) = 20GB + 10GB = this 16bit .psb layer file would need 30GB RAM, so when i have 32GB in my imac, i set cs6 or cc to 70% ram usage, it misses at least round 8-10GB RAM > can i guess that photoshop would swap these 8GB onto my scratch disc? or do i miss something important in my thinking?