I usually leave about 10% free space, but some say up to 20% is best.
Make sure to optimize your drive by turning off hibernate, disabling system restore and if you have at least 8gb of ram (and don't run VMs or any real memory intensive apps) either turn off, which I don't recommend, or as I do recommend move your paging file to a HDD as it will rarely be used and just wastes space.
Friend, this is not how SSD works. They operate by firstly filling any unused cell, wich means that it will certainlly and for sure utilize 100% of its space way before it gets close to getting full. What happens is that it will only delete/overwrite deleted files after the SSD is already full. This is done so the bridges are used equally among the entire SSD