The bonding to the substrate is fairly consistent. Note that the type of thinset you use depends on what the subflooring is: if it's a wooden subfloor, it must be a modified thinset, otherwise, it can be an unmodified...they are quite specific in their installation manual and it behooves you to follow it for best results.
The top, setting tile, is more problematic. The more irregular the subfloor or inconsistencies in the tile will call for more thinset to ensure that you get a nice flat tile installation. And, they specify this thinset must be an unmodified variety. So, you may need two different varieties, or an unmodified with an approved, add-in modifier, or separate varieties.
How fast you work, and how much wastage you have from maybe not using all of the thinset prior to it starting to set up may also have an effect on your coverage. Also, how you hold the trowel can make a big difference in the actual thickness of the applied thinset. Hold it at 90-degrees will use more than holding it at say 45-degrees. So, estimates are just that.
In the scheme of things, the stuff isn't all that expensive...depending on what your time is worth, having an extra bag or two probably shouldn't be a deal breaker. Also note, the stuff you buy will have a manufacturing date code on it, or a use by date. It does potentially go bad - you don't want it to be close to the use by date when you just get it - fresher is definately better. If the bag isn't sealed perfectly, or it has a small hole, or it is improperly stored can all impact the stuff...portland cement just loves to absorb moisture, and if it does, it may not make the bag a solid, but it may end up being a lot of cured 'sandy' particles, that won't adhere to anything.
The guys over at www.johnbridge.com use a lot of Ditra, you might ask there.