Borosilicate or Neoceram/Pyroceram glass is your best bet for high-temp materials like ASA. Just a heads-up - tempered glass can shatter under thermal stress, and trust me, it’s not fun to clean up after. 
The downside?
- Pyroceram is heavier
- Borosilicate at K2 bed size can get a little pricey
That said, a 0.7mm deviation really isn’t bad! Totally manageable with proper prep. In fact, I just finished up a full bed leveling guide using Kapton and aluminum tape to dial in high-precision meshes - my K2 is down to a 0.056mm deviation. I’m planning to have it up on YouTube and our forum sometime this weekend.
You can check out some of the highlights here:
Dialing in the K2 Plus to a 56 Micron Bed Deviation – The Madness & The Method
Let me know if you want help tuning yours - I’ve been living in mesh view for weeks. 