For a steam surface condenser - air or water cooled - the functional parameters include: condenser heat transfer surface area, cooling fluid temperature & flow rate, steam enthalpy & flow rate, (surface conditions - fouling), (amount of non-condensibles present),...
Neglecting surface condition and presence/absence of non-condensibles, the relationship of condensing pressure vs. steam flow typically is a smooth curve, or series of curves for discrete values of cooling fluid inlet temperature. There is no "critical point", so to speak, in the neighborhood of typical operation of surface condensers.
I think you would be able to obtain a generic condenser performance curve from a condenser manufacturer, or their website(s). Graham Mfg. and Ambassador Heat Transfer Co. come to mind for water cooled surface condensers.
Try a keyword search ("condenser", or "steam condenser"
from the top of this web page; a post (by MJCronin) had recently cited a new reference book on surface condensers. I don't recall the details, but you should be able to find the thread containing the post - sometime in the past 3 months I think.
