I suppose you are looking for an evaluation of the steam consumption required to ensure deaerator does its job.
First of all consider the aim of this device is to scrub oxygen from feeding water a very in order to prevent heavy damages to the boiler. The principle exploited here is the decreasing oxygen solubility in water with temperature increase. At temperature of 104 °C @0.2 bar(g) pressure water is practically free of dissolved oxygen.
The amount of steam required is that to heat up the make-up water to its saturation temperature (you’ve always to feed the boiler with fresh water at least just because of blowdown procedures), plus the amount of steam vented with non-condensable gases less flash steam from condensate.
I have an interesting note on DA (Italian language): because of your handle, perhaps you could be interested. Let me know.