There is no good formula for the lowering of AeT pace as you go up in altitude. We can only say in a qualitative way that it will drop and we have much anecdotal evidence from climbers recorded HRs at high elevation. I discuss this in some detail here
Note that David’s HR was about 20 beats below his low elevation AeT for this climb at 6100m despite the perceived effort being higher than AeT at sea level. Assume some thing similar for yourself.
I hope this helps.