If its like the two GFs I had standing room tickets for in 02 and 03 you'll be in a particular standing area, ie at the back of and behind sections M4, M5, M6 etc. As far as I am aware there will be a certain number, and equal number, of 'tickets' in each area. You cant go and choose the area in which you want to stand so whatever one you have is where you will be.
In 02 we got there about 12pm and had to stand about 3/4 of the way back, was OK for me and my mate because we're 6ft 1 and 6ft 3, but if you're way shorter than that I would be suggesting getting there a tad earlier.
Of course people move throughout the day and you'll probably find your spot moves sideways, backwards or forwards throughout the course of the game, but most people, once they have their spot (especially if its towards the front) will be hellbent on getting that position again when they return from wherever they have been.
I'd be getting there reasonably early - from what I see the Unders is on Friday night at TD so not sure whats on before the pre-match entertainment, possible super-soakers...