Condensate is distilled water at a couple of hundred degrees that you have already bought, already treated and already heated. Every kilogram that does not come back has to be replaced with cold, raw, chemically hostile makeup. On a steam audit the return percentage is the first number anyone asks for.

$\%CR = \dfrac{100\,S_c}{S}$, where $S$ is the steam production rate in kg/h and $S_c$ is the condensate returned, in the same unit. $\%CR$ is the condensate return percentage. A well-run industrial plant returns 70 to 80 %; a plant injecting steam directly into a process returns almost nothing, and honestly cannot.

$M = S\left(1 - \dfrac{\%CR}{100}\right)$ turns that straight into the makeup rate $M$, also in kg/h. The bracket is the fraction that did NOT come home. Take the complement by mistake and you have calculated the returns instead — at 75 % return the two answers differ by a factor of three, and one of them is the boiler house's water bill.

Then the loss on the way back. When hot condensate passes a trap into a lower-pressure line, it is carrying more sensible heat than water can hold at the new pressure, and the surplus boils part of it off instantly. That is flash steam: $\%F = \dfrac{100\,(h_{f1} - h_{f2})}{h_{fg2}}$. Here $h_{f1}$ is the liquid enthalpy upstream, at the higher pressure; $h_{f2}$ is the liquid enthalpy downstream, at the lower one; and $h_{fg2}$ is the latent heat of vaporisation downstream — all three in kJ/kg from the steam tables, and two of the three belong to the low-pressure side. The subscript $f$ means saturated liquid, $g$ saturated vapour, and $fg$ the gap between them.

Read the fraction as an argument: the numerator is the heat the water is no longer allowed to keep, and the denominator is the price of turning a kilogram into vapour. Typical answers land between 8 and 16 %, and that flash is not waste unless you waste it — vented off a receiver it is a plume in the yard, and piped to a low-pressure main it is free steam.