While the previous advice is a true way of filling your reel with the optimum amount of backing, it is usually more of a PIA than it's worth for backing you may never see again until it's time for a new line. The only time I have seen my backing on my 4wt is when I hooked a 15lb carp last summer. Even then I only saw maybe 30yds of it. An easy way with alot less stress is to fill your reel 1/3 to less than 1/2 full with backing. Then put on your line. If it turns out to be too much it's alot easier to take off 90' of line and a little backing then to remove 100 - 200 yds of backing plus line, turn it around and start over.
Just my 2 cents.
Enjoy your new gear!