Are the -1 target values coming from the TD Boost section? It may be best to ignore this section as Target and Actual are 2 different things and it can get confusing since Target does not mean the car will actually achieve it.
At idle the negative 8 reading is correct as that corresponds to 16 inches of mercury.
Next step for boost pressure settings
If I read the post correctly, the car only fluctuated with stage 1 and has not done so with stage 2. If this is the case then stage 1 was over boosting, whereas Stage 2 needs more waste gate duty cycle. I'd try running the HWG map and see if:
1- the car achieves close to target boost (+/- 0.5 peak psi is acceptable)
2- the PSI fluctuates up and down
If the car only achieves 15psi then I would shorten the arm by 1.5 turns. That should net ~16.0 PSI.
Keep us updated as you find out more