15 minutes with 7 minute And 11 minute sand hour glasses

I have two sand hour glasses:
1. A 7 minute one and 
2. An 11 minute one.
Using just these 2 sand hour glasses, how can i measure time as 15 minutes ?

1. Start both the 7 minute hour glass & 11 minute hour glass.

2. Wait till the 7 minute hour glass times out. Time is 7 minute!
3. Restart the 7 minute hour glass. At this time 11 minute hour glass will have 4 minutes left to time out.
4. As soon as 11 minute glass times out invert the 7 minute hour glass. Total time now is 11 minutes.
5. After inverting 7 minute hour glass, it will now have 4 minutes left for time out.
6. After these 4 minutes times out, the total time is 15 minutes.