Cut and fill depth categories

I have encountered a similar limitation that I can imagine would be useful to others as well:

I have been quantifying asphalting depths for a runway which is approx. 120,000m2. The difference between the existing runway surface and the proposed varies (i.e. thicknesses to be asphalted are between 0 and 180mm). The asphalt needs to be laid in given layer thicknesses to meet the specification. As an example the surface course we are using can't be laid and compacted at less than 40mm thick. This leaves the area of the runway that needs to be asphalted (between 0-39mm thickness) that we will need to plane out so we have the required depth. I can visualise this by adjusting the colours and being specific with the depths of the contours but I can't get an area or volume of the different contour bands. I need this because I need to quantify how much of the 120,000m2 needs to be planed out (i.e. is this 10% of the total area, 20%, 30% for instance)

So far I have exported the data into CAD and traced round the contours to get areas of each different contour band, but that is time consuming. I have been given a workaround which will involve some manual calculations, which should work but for the benefit of others that have the same problem in the future it is a feature I would like to see.

The other thing that I am unable to deal with is layers that have a thickness of 15, 25, 35, 125mm for instance. Most of the time for earthworks etc. the 5mm is negligible but it would be nice to be able to increase the accuracy if needed.

Thanks.