Garmin Edge Elevation Setting?

[cosmicbike]

Just upgraded my Garmin 200 to a 500, and I'm intrigued by the elevation function. I get that it determines height variation itself (rather than rely on GPS info like the 200 does), but wondering if anybody sets their start point elevation, so you know how far above sea level your ride takes you, rather than just knowing how much you have climbed/descended?
If you do, where do you find your start point height ASL? I've tried a few sites including Google Earth, but my start point varies from 17m - 49m ASL...

Thanks

 

[Albert]

My home is 780ft above sea level. I went to the seaside, set my Garmin to 20ft (standing on the Prom) and went the 12 miles home. I have repeated this a few times, so I know that my number is basically correct to within a very few feet.

 

[Tanis8472]

Try this http://www.altitude.nu/

 

[HLaB]

I used to set the elevation level of my old Garmin Geko but I never bother with the 800, I usually reset it about a mile or two fro the flat to record the proper ride and its's usually sorted itself out by then. On the rollers though I have the compulsion to set the elevation to 0ft

 

[cosmicbike]

Thanks folks, I got 55ft from the link @Tanis8472 provided, which perhaps unsurprisingly ties up with the Google Earth value.