It takes 15GB of my SSD space when I open Photoshop!

RAM and scratch disk are separate things. Some operations use a lot of scratch disk , some use RAM. The 8GB RAM minimum is just that. If you only had 8GB scratch disk space free you would quickly hit an out of scratch space error.

I have 64GB RAM installed here but still just after opening my scratch file is 5.3GB . I often see it run up around 12-15GB occasionally much more.

Dave

It may be for you environment. How much scratch space is used when to start  Photoshop varies from user to user. It depends one how you configure Photoshop,  how many patterns you have added, how many  add-on you install etc.  15GB seem large to to me.  I see Photoshop start with 5.5GB.  Your in better shape then me.   After I start PS  I'm down to around 120GB free on my SSD.  I have 40GB of  Ram Photoshop only uses around 1GB to start.   I have see PS use all my RAM when I run some scripts.  Most of the time  PS does not use more than 10GB of ram. I have seen Photoshop use 100GB+ of Scratch space.  So I thing your are safe with 175GBG free. Adobe recommend 8GB of RAM I think 16GB would be better.