Let's say bitcoin ATH is $7800 and it plunged to a low of $5507 which mean:
$7800-$5507 = $2293 (decrease)
% decrease = $2293 ÷ $7800 × 100
% decrease = 29.39
decrease = 29.39%
Thank you for your time and effort error08! This is exactly what i was looking for.