[sdiy] Help with math, please

[Olivier Gillet]

accumulator += (target_value - accumulator) * Ts / RC

Where Ts is the sampling period (the interval between each sample) and
RC the required time constant (ie, for slow envelopes, you have a very
small k) ; under the restriction that RC is well above Ts to avoid
stepping.

You can look at it from both the "digital filter" and the "discrete
solution to the differential equation of the RC system" angles.