Simplify margin formula
Passed STC non-reg:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 46944 W: 11934 L: 11730 D: 23280
Ptnml(0-2): 95, 5403, 12273, 5605, 96
https://tests.stockfishchess.org/tests/view/6a0b018d6524d21ee79b8680
Passed LTC non-reg:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 535980 W: 135398 L: 135702 D: 264880
Ptnml(0-2): 241, 58385, 151023, 58119, 222
https://tests.stockfishchess.org/tests/view/6a0c3ab86524d21ee79b87f2
Passed VLTC non-reg:
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 126632 W: 32068 L: 31961 D: 62603
Ptnml(0-2): 14, 12466, 38244, 12583, 9
https://tests.stockfishchess.org/tests/view/6a1ac130818cacc1db0ad03c
closes https://github.com/official-stockfish/Stockfish/pull/6880
Bench: 3493826