Mnemonic for the tone split: third-tone
数 is the ACTION (fingers dipping as you tally — the tone dips too); fourth-tone
数 is the RESULT (the final number lands hard). Idioms like
屈指可数 ('few enough to count on one hand') and
如数家珍 ('reel off like family treasures') keep the third tone because they're still about the act of counting.