The decay of 3X ETF due to price oscillation can be simply calculated:
GDX=100, up 10%, =110, down 10%, =99, up 10%, =108.9, down 10%, =98.01, up 10%, =107.81, down 10%, =97.03
NUGT=100, up 30%, =130, down 30%, =91, up 30%, =118.3, down 30%, =82.81, up 30%, =107.65, down 30%, =75.36