Usually when playing the 2 necron players i play they count being less that 25% of their entire army when phasing out.

Which units do and do not count towards the points to calculate when phasing out?

Anything that is 'Necron' counts to phasing out models. So any model that can roll for WBB counts as being 'Necron'

From Memory:
Necron:

Warriors
Immortals
Lord
Flayed Ones
Wraiths
Destroyers
Heavy Destroyers

Not Necron:

Pariahs
Scarabs
Tomb Spiders
Monolith
C'tan


So basically everything that is important is 'Necron' as far as I am concerned. I rarely use non 'Necron' models as I fid they don't give me value.

So if the guy has 1500 points of Necrons looking something like this:

1 Lord with Weapon = 110
36 Warriors = 648
10 Pariahs = 360
3 Wraiths = 123
3 Destroyers = 150
2 Tomb Spyders = 110

Total = 1501

Total 'Necron' Models = 43

Therefore you would need to Destroy or have lying down at any given point 33 'Necron' models.

I will highlight why the 'Necron' part is so important:

You could do:

1 monolith = 235
10 Pariahs = 360
2 Tomb Spyders = 110
1 Lord = 100
1 C'tan = 300
22 Warriors = 396

Total = 1501

Total 'Necron' models = 23

Needs 18 to destroy entire army. But more importantly, you would only need to destroy 324 points of this army to kill the entire thing :) This means you would have 1176 points worth of models still on the table but would have lost... Scary huh?? :D

Okey dokey that is a really good thing in one way but yes very scarey. that would really suck but there a perks and disadvantages of every army in the game. But this is different, with the list you displayed (iornicly close to my mates list) the crons would go down without much of a fight.