It may disappoint you slightly to learn that Garmin elevation is hugely innaccurate. Spot elevation is fairly good (ie. when you're stood still), but I've found it to be as much as 100% out on runs before. For example, a hill reps session off road last year I worked out from a map at around 2000ft total (250ft/rep x 8) and yet the Garmin read around 4200ft!