If the arrow doesn't flex properly when shot, it will have a tendency to hit left or right of the target. Your fletching is what helps straighten out the shot. However, a broadhead tends to resist the help fletching adds. Therefore, it's good to get spine correct. This way, your arrow has the maximum forgiveness built into it. This can make a huge difference in wind or if you hit a small twig, leaves or grasses.

The best way to design your arrow is to pick an adequately heavy tip. I can't recommend less than 125 gr for any setup and I firmly believe most (especially beginners) will benefit from much heavier tips. After you determine tip weight, it's fairly easy to get into a spine range from mfgr charts. Pick a slightly stiff arrow. Then you start by building one at an inch or two longer than you expect to shoot and then bareshaft test it. By trimming 1/4" at a time from the shaft, you can make this bareshaft fly perfectly (with a field tip) to your target. At this point, you know you have the spine correct. Hopefully, at the same time you choose the heavier tip which will give you a higher FOC, which adds to stability and penetration of the arrow.

If this process is confusing, research it and study it until you learn it. It's by far the most important aspect of tuning a hunting setup. Keep in mind, judging the quality of your arrow flight with field tips can't be done with fletching attached.