No problem.
Since you're using a pack of string, you'll have nothing to worry about (even a poly).
You don't even need to go through all that rigmarole that Irvin is suggesting.
The pack will be more than enough, considering the Phantom 100 is a 100sq.in. hoop with only a 16x18 pattern. A cakewalk, even considering the additional length typically required when using a rotational gripper.
Cut 20 feet from the pack for the mains. Use the remainder for the crosses.
My advice - measure exactly how long that particular pack of string is (don't assume that it's exactly 40 feet). Know exactly how long it is before you begin.
Then, when tensioning the last main, on each side, measure exactly how many inches are extending beyond the back of your gripper. Write down the excess amount (better yet, keep a detailed stringing log) Then, do the same for the top & bottom crosses. Write down any excess amount(s).
This will allow you to calculate the amount you truly needed. Just subtract the excess from each of the lengths that you originally began with.
This is helpful information to have later on. Especially if, down the road, you're cutting string from a reel (or just stringing another 16x18 frame with a ~100" hoop).
BTW, do you have a starting clamp?
If yes, do you happen to have 2 starting clamps?
@Yamin, I would strongly dissuade you from mounting your frame "offset" the way Irvin has suggested.
I won't rehash the debate that took place, recently, in another thread.
If you need further reading/explanation see the following post, especially what I wrote in post #27, #31 and #33.
Even IF you wanted to attempt what Irvin proposed that you do, I'm not sure it's even physically possible.
To accomplish the ridiculousness that he's suggesting (mounting the hoop severely "offset") you would need to shift each of the mounting towers far enough that the turntable pivot point would need to be higher (within the hoop) than where the 3rd cross is going to reside.
I seriously doubt that the towers can even be "offset" far enough to accomplish this feat.
In my opinion, it's an absolutely asinine idea to even suggest.
