The formula is:
NB-(NB*(0.60^(1/12)))
This equates to reducing the balance by 40% each year, instead of the straight line method.
For an Asset purchased at $21,600 and depreciated monthly over 36 months, the first five entries would be:
$20,669.81 (depreciated down $900.19)
$19.837.14 (depreciated down $862.67)
$19,010.42 (depreciated down $826.72)
$18,218.15 (depreciated down $792.27)
$17,458.90 (depreciated down $759.25)