The question is:
1. My home point altitude is 1.000 meters over the sea
2. The flight is downside till 750 meters for example, so 250 meters BELOW the home point
3. The failsage altitude choosen in DJI GO app is 50 meters

4. In case of failure, what will be the altitude the Phantom 3 Pto will reach before returning home:

    ==> 1.000 + 50 or 1.050 meters ?

    ==>    750 + 50 or 800 meters high, in that case not sufficient to come back safely
5. In other words, the failsafe final altitude is ihe home point altitude + the altitude defined in the DJI GO app to return or
6. The altitude reached at the moment of the failure + the altitude for failsafe procedure defined in the app ?

Thank you all for answers

Excellent question, I had the same.

I would say: RTH setting (your example 50m) + Home point altitude = climbing to 1050meters in case of RTH.

If you fly lower then you home point you see -250m. In case of failsafe the P3 is climbing 300m

The default RTH height setting is 30m, if you fly off a cliff and descent 100m and lose signal your aircraft will ascend to 130m.
It doesn't matter how high you are above sea level, your home point is always 0m

SHamers Posted at 2016-1-9 13:53
I would say: RTH setting (your example 50m) + Home point altitude = climbing to 1050meters in case o ...

Many thanks.
That's also my understanding, or my hope.
Many thanks.
That's also my understanding, and hope.
Do you believe the app take this eventual total climbing of 300 meters in consideration to calculate the correct moment of RTH with enough remaining power ?

Jacques850 Posted at 2016-1-9 16:03
Many thanks.
That's also my understanding, or my hope.
Many thanks.

It does take it into account when calculating RTH power requirements.  The further away it flies the further to the right the home position moves on the remaining flight time scale, including the further down or up it flies.  The only thing it can't calculate is if it suddenly finds itself battling a change in wind direction/strength when flying back.

nigelw Posted at 2016-1-9 14:24
It does take it into account when calculating RTH power requirements.  The further away it flies t ...

Thank you a lot

DJI-Ken Posted at 2016-1-9 13:59
The default RTH height setting is 30m, if you fly off a cliff and descent 100m and lose signal your  ...

Thanks Ken

nigelw Posted at 2016-1-10 00:24
It does take it into account when calculating RTH power requirements.  The further away it flies t ...

Excellent points, Nigel! Thanks.
DJI-TIM adds a few more gotchas in his thread @ RTH-How It Works and What to Avoid
And for those of us non-metric minorities, remember that RTH is always in METERS!!!!
DJI-TIM also produced an initial animation here that is clever and on the topic @ What to do after RC signal is lost

And for those that need yet another RTH graphic

terrylewis Posted at 2016-1-9 14:50
Excellent points, Nigel! Thanks.
DJI-TIM adds a few more gotchas in his thread @ RTH-How It Works  ...

Unbelievable.
Fantastic and very clever answer.
I'm absolutely grateful.
Thanks all

terrylewis Posted at 2016-1-10 00:50
Excellent points, Nigel! Thanks.
DJI-TIM adds a few more gotchas in his thread @ RTH-How It Works  ...

Thanks Terry for posting the link,
I should have posted that link, it's an excellent detailed explanation.