Q: How is room volume in "Quick Estimate" calculated and how does it influence reverberation time?

A: There are different ways to estimate/describe volume in "Quick estimate".
If the "User defined volume" box is not checked, Odeon will perform an automatic estimate of volume using the formula for mean free path taking into account whether 0, 1 or to 2 sides of each of the surfaces are visible from within the volume where the source resides (this is detected by means of ray-tracing). So if a surface is visible from within the volume its area will be counted once or twice. If only a part of a surface is visible from within the volume then the surface area of the room is overestimated (e.g. if the floor surface is much larger than the floor visible from within the room) - leading to an incorrect volume estimate when applying mean free path formula: V=L*S/4. For very complex coupled volumes it is debatable if it makes sense to talk about one volume (and about a global reverberation time as well) – even though the estimate provided by Odeon might be fairly precise. When “user defined volume” is checked, the number in the box to the right will be used as a basis for estimating the Quick estimate reverberation times.
Another way to make Odeon help you guessing the volume of the room is to press the suggest box volume. This box volume is a box defined by the outer dimensions of your model (length-with-height) also given if you press the info button or (Shift+ Ctrl+ R). So you will see that this suggestion does not change if you use different sources as the estimated volume will.
If you know best you can enter a box volume from your own calculations and click the user defined volume to calculate the reverberation time.
Remember that the Quick Estimate only gives a fast estimate of reverberation. Better estimates are given by Point response calculations in the "Job List" and "Global Estimate" which does not assume diffuse conditions and indeed does not depend on volume at all.