My tune safe for Nitrous?

[Powerhouse]

Logged a run today without nitrous... at WOT timing is around 21 at 5600rpm... when I let off, it goes up to around 37 38 until I go WOT again. Is this normal?

newb.

[Boost]

I would also like to know how much timing is safe on my '07 Silverado with a wet 75 shot. (4.8 liter "C" motor)