How to specify HALT / HASS limits?
I would call it an art rather than a science, in my experience you start developing the screen with very little heat and vibration and gradually increase temp and vibration independently until failure occurs or you hit predetermined or reasonable limits. You combine the two (vibe and heat) to run a sequence of cycles at the maximum determined limits (say 2 or 3 Hot/Cold cycles with vibration) as a HALT

The HASS profile is determined from the results of the HALT. Typically you pull back the profile across the board and run a Proof of Screen to verify that the HASS is not significantly damaging or impacting the lifetime of the unit. 10% on heat and 50% of vibration is the rule of thumb that I have used in the past.

The difference between HALT and HASS is important to understand. In HALT you are testing the lifetime of the device under accelerated conditions, the goal is to find that limit so you are expecting units to fall out. For HASS the intention is to screen for premature failures and defective units. Say that 95% of your failures happen in the first 5% of the lifetime (or warranty period :P ), a good HASS will accelerate that first 5%  in order to catch those premature failures before they leave the door. The art is in determining based on the HALT (which will run through a larger portion of the lifetime) the proper profile so that the non-defective units will function normally for the remainder of the lifetime.

There are different philosophies towards testing, but this is the one I was taught by a lab that does testing for tesla, google, apple, etc. products. 

To summarize:

HALT: Push known non-defective devices to failure limits (doesn't need to explode, a recoverable failure to function is enough)

Proof of Screen: Reduce the profile and verify that non-defective units do not acquire damage from the screen. Known defective units should be dropping out at a reliable rate

HASS: Run your production units (or a portion of them), with confidence that you will catch a majority of deffective premature failures and that you are not damaging properly assembled units