@Inna22I like to think in percentages for this kind of inconvenience. As @Kelly149 advises, begin by making sure that the issue is not user error, then ask yourself what a reasonable percentage of the stay has been disrupted by the non-working amenity. Is it 25% of the 2 night stay? Is it 10? 30? What would YOU find reasonable if it were your stay?
Once you have a percentage in your mind, ask the guest what they think a reasonable % would be. Assuming they are making a reasonable request, negotiate from there. Percentages have a way of equalizing things. As a reasonable guest, I might think 25% is fair (well, it's only the oven, but the heat works, and the water is fine, and the wifi is fast, and the apartment is lovely, so 25% seems ok), where I might look at $75 (300 x 25%) and NOT think it's fair (Geez, I spent $40 on groceries alone...)
Keep in mind, though that CX will refund 50%, I think, for a non-functioning listed amenity (which I think is really high, depending on the amenity). I guess the goal is to avoid that 50% penalty and try to settle on a lesser percentage that still makes the guest feel compensated.