Tweak pruning formula
Tweak pruning formula, including a constant. I started from an old
yellow patch, if I'm not mistaken by Viz (Unfortunately I lost the link)
where he tried something similar.
I worked on it, trying different variations, until I came up with a good
configuration to pass.
Passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 213120 W: 55351 L: 54778 D: 102991
Ptnml(0-2): 572, 25209, 54437, 25758, 584
https://tests.stockfishchess.org/tests/view/6660c9a7c340c8eed7758195
Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 315324 W: 80176 L: 79284 D: 155864
Ptnml(0-2): 155, 34711, 87030, 35619, 147
https://tests.stockfishchess.org/tests/view/6660d7bb6489614cdad13d66
closes https://github.com/official-stockfish/Stockfish/pull/5370
Bench: 1231853