Photoshop is showing 100% cause it has all the RAM it needs for processing... it's up to you whether you want to create a RAM disk scratch or not... if the scratch disk just records history states, having it on HDD probably not gonna slow down your workflow an awful lot compared to RAM disk or SSD, and SSD would probably be not worth the expense...
a RAM disk scratch would be a greater help for 32bit Photoshop, which is not your case, where the program can use much less RAM and more data needs to be written in scratch... even then, if one is only using like 2GB of RAM, then even for 32bit Photoshop, the program would be able to access enough RAM...