emwaving like a boss
I reached a ceiling after practicing through 100 emwave sessions: 97% in high coherence at the "highest challenge" level for 10 min.
I found that a very regular breathing pattern is enough to achieve this, nothing like having to think happy thoughts or all the "woo-woo" visualising-your-heart stuff 
This also highlights a bug in the v2.0 of the software for the calculation of the "Longest Singular Duration Spent in High Coherence". It reports 7 min. But when I look at the ZoneScore in emwave.emdb ( 0 = low coherence, 1 = medium coherence, 2 = high coherence ), out of 122 measurements for that session, only the first 4 values are 0 or 1, meaning the duration spent in high coherence was really 9 min 40 s, not 7 min.
However the "longest singular duration in high coherence" measure has been removed in the more recent versions (3.x) of the software.
On a sidenote, Dave Asprey, a certified Heartmath coach, managed this only at level 3 (high, not highest) for only 5 min .
What now? It is time to try emwaving under much more stressful situations. Stay tuned!