every little bit(e) counts

Average human body weight (global): 62.0 kg (ref)

Average human little finger weight: ~10g
I have no good reference so this is my pinky volume times avg human body density. Probably a garbage estimate.

Average human little finger contribution to body weight 10g/6200g=0.0016 so 0.16%

Body weight has different contributions to the basal metabolic rate (BMR) for different ages and sex. For the sake of a ballpark figure I take the Katch-McArdle Formula (which is the worst one probably):

BMR = 370 + 21.6 * (1 - F/100) * W
where:
W is body weight in kg.
F is body fat in percentage.

Let
A = 21.6 * (1 - F/100) * W
Then the absolute change in BMR is:
ΔBMR = d * A
->-0.0016 * 21.6 * (1 - F/100) * W
And the percentage change (account for the added 370 constant):
%ΔBMR = -0.16 * A / (370 + A)
->-0.16 * 21.6 * (1 - F/100) * W / (370 + 21.6 * (1 - F/100) * W)

W is body weight in kg (pre-pinky-off)
F is body fat in percentage (pre-pinky-off)

Placeholder values can be something like 30 for Women and 20 for Men.