Calculation date and margin of error are displayed when you let the cursor over a watt value.

The beginning of Risoul climb was a quite low slope (6%), ridden at 28km/h behind a AG2R team-mate. Consequently, the estimation cannot be done on this section (too much drafting).
The choosen section starts in Risoul, and ends at the finish line.
Majka's performance couldn't be measured on that section. But it was possible on a shorter section while he was alone (9.66 km, 25'42", 426W, 6.2W/kg), perfect analysis conditions.
For the full climb (from Guillestre), the times are (+/-5s) 31'50 for Nibali-Peraud, and 32'15 for Pinot, Bardet, Van Garderen and Majka