Generally most of the steam is recycled as condensate to the boiler but still there are some losses and need some make-up water for boiler. Is there any rule of thumb for these steam losses.?? It also depends of the amounts used for stripping operations, on vacuum ejectors, steam tracing discharging to the atmosphere, burner steam atomization, fired heaters velocity steam, vaporization from condensate collecting tankage, etc.
Many years ago, much before energy costs started to raise, I used to work for an oil refinery in which the make-up was at the 40% level. 
The blowdown rate can be computed using this formula:
BD = (F*S)/(B – F)
Where:
BD = blowdown rate (kg/h), to be restored through makeup water
F = Feedwater TDS (ppm)
S = Steam production (kg/h)
B = Required boiler water TDS (ppm). This  control valve depends on the specific type of boiler installed.
You may also want to take into account steam leakage losses

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