Add or increase darkvision

[Dastir]

I have spent some time searching the forums, but could not find the answer ot this question...

I am trying to add a prestige class that grants Darkvison at level 1. if the character already has Darkvison, it increases the range by 30 feet. I am at a loss as to how to accomplish this.

I copied the ability from the Liberator class which says it grants 60 or increases by 60, but it does not actually increase the range - the specials column on the sheet still shows 60 feet.

I tried the following script:

var exists as number
exists = hero.pickexists[rDarkVis]

if (exists > 0) then
   hero.child[rDarkVis].field[Value].value += 30
else
   hero.child[rDarkVis].field[Value].value = 60
endif

But since the ability boostraps Darkvision, it is always setting the range of the darkvison to 90 feet, regardless of whether the character already had darkvision or not.

Help?