How much HD space needed for memory swapping

(I hope that "memory swapping" is the correct term.)
Model:    Late 2013 Macbook Pro
OS:    OS X 10.13.6 High Sierra
HD:    1 TB, 954.34 GB used, 44.27 GB available
CPU:    2.6 GHz Intel
Mem:    16 GB 1600 MHz DDR3
As you see, my HD is very full.
As it fills up, I have seen a big dropoff in function -- programs load much more slowly, can only have a few programs open at the same time, windows don't open at full size, etc.
I assume this is because of not enough swap space on the HD.
What is the recommended amount of HD space to keep free for good performance?
Thanks!
[NOTE:  Originally misposted to comp.sys.mac.apps.]
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Dudley Brooks, Artistic Director
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