Try this. The purpose of defining today as a fixed date is to be able to try different values for total to test the formula.
var date Today = Date(2019,1,15);
var date EffectiveStart = FirstDayOfMonth([Start Depreciation]);
var date EffectiveToday = FirstDayOfMonth($Today);
var date ES = $EffectiveStart;
var date ET = $EffectiveToday;
var number MonthsStepOne = (Year($ET) - Year($ES)) * 12;
var number MonthAdjustment = If(Month($ET) >= Month($ES), Month($ET) - Month($ES) +1, // both ends count as a month, so add 1
(Month($ET) - Month($ES)) + 1);
var number Months = $MonthsStepOne + $MonthAdjustment;
Max(0, [Original Amount] - [Original Amount]/[Depreciation Term (months)] * $Months)