fireflybob, this is Rob, from SkyDemon.
Controlled airspace vertical limits are defined in the AIP by their FL, so there is no need to derive them.
GPS provides the position of the aircraft, and with multiple position reports we can derive speed and direction. SkyDemon then looks at the absolute position of the airspace limits to see if continuing at that same speed and direction would intersect the airspace within the warning time set by the pilot.
Hopefully that answers your question, but if not then please PM me to go into more detail.