When to rebuy/ad-on

[spliff]

Hi,

I am going to play a live-tourney in 1,5 hour.

50$, with 1 rebuy (when under 5000 chips) or 1 ad-on after one hour (regardless of stack-size)

Expect about 150 players, stack: 5000 chips.

There is a screen where you can see average stack.

Does anyone have pointers to

1: When to make ad-on (after an hour) in relation to average stack ?

2: If going broke/low when to make rebuy in relation to average stack (# of players) ?