Hot condensate under pressure is holding more sensible heat than it can hold at a lower pressure. Open a trap into a flash vessel and the surplus goes somewhere: part of the water instantly boils. That is flash steam, and it is free steam if you catch it and lost steam if you do not.

The fraction that flashes is $F = \dfrac{h_{f1} - h_{f2}}{h_{fg2}}$, and multiplying by 100 states it as a percentage. Subscript 1 is the high-pressure side, the main; subscript 2 is the low-pressure side, the vessel — that convention holds for the whole lesson. So $h_{f1}$ is the liquid enthalpy upstream in kJ/kg, $h_{f2}$ the liquid enthalpy downstream, and $h_{fg2}$ the latent heat downstream. The numerator is the sensible heat the condensate can no longer keep; the denominator is the price of one kilogram of vapour at the new pressure.

That denominator is where the marks go. The latent heat is read at the LOW pressure, because that is where the boiling happens. Reading it upstream is the named mistake, and it flatters the answer by a percent or two — small enough to survive a sanity check, which is exactly what makes it dangerous.

Then the rate is simple bookkeeping: $\dot m_f = \dfrac{F}{100}\,\dot m_c$, where $\dot m_c$ is the condensate rate through the trap in kg/h and $\dot m_f$ is the flash steam rate leaving the vessel, same units. Estimate first: flash fractions live between about 5 and 20 %, and that rail catches an inverted ratio before the arithmetic does.