Ok, typically we would want you to do more of it yourself, but there is enough SWAG in this that I'll have to help some anyway...
So here's an SI psych chart:
http://www.uigi.com/UIGI_SI.PDF
There are two major complexities about your scenario:
1. How much moisture is gained/lost through the walls?
2. How much moisture is lost through exhaust/infiltration?
I'm assuming by "bunker", you mean concrete, underground, with no insulation or vapor barrier. Typically such bunkers are very humid because of moisture infiltrating through the concrete (like a basement). But if you are exhausting from it, you'll also be pulling in outside air, potentially at roughly zero humidity in winter. The infiltration rate you gave is small relative to the room volme (about 0.4 air changes an hour), but not negligible.
So my assumption will be that all of the added moisture is used to counteract the dry air coming in from outside.
20C @ 65% RH is about 9.5 grams of water per kg of air (from the chart). 0.25 m^3/sec is 0.3 kg of air per sec, so that's about 10 kg of water per hour that needs to be evaporated to counter-act the dehumidification of the entering dry air.
Of course, all bets are off if my assumption about infiltration through the walls is wrong. Indeed, for best results, you want a push-pull ventilation system with a
positive airflow balance (supply airflow larger than the exhaust airflow) to eliminate other sources of infiltration.