If either method has already been mentioned here, I missed it...
One caveat: These methods requires extra safety!
Since both methods require a cocked TAC 15 with an arrow loaded. I would suggest clamping TAC 15 close to a back stop to where you can't get your body parts (head included) between the pointy end
of the arrow and the back stop. Double check to make sure the Safety is On. Test that the Trigger can't be pulled.
When adjusting the arrow rest up/down or left/right. Remove the arrow!
Yes, you may have to make a few additional estimated adjustments, but it is worth the safety.
Now to the methods:
You can use a Dial Caliper (1/100th of inch or better) to measure the height of arrow close to front of TAC 15 and close as possible to picatinny rail (scope mount) end of TAC 15. The height of arrow at both ends should be equal. I measure from top side of arrow to floor of the cocking shuttle channel.
You can also use a Digital Level, if the level is accurate to a tenth of a degree. However, it won't be as accurate as using the Dial Caliper (1/10th degree roughly equal to 0.04 inches in this case). The TAC 15 does not need to be perfectly level. You just need to raise or lower the arrow rest to where incline of the arrow matches the incline of the TAC 15 body (edge of cocking shuttle channel).
That takes care of the up & down. The left and right is a little more tricky to measure. At least with the dial caliper I have. But still its the same principle measure distances between side of arrow and body of TAC-15 near the front and near the rear.
I have not tried the Digital Level method for side to side center shot setting. Since I have a drop-down rest, instead of Whisker Biscuit. But it should be possible to strap (rubber band) the arrow to where it stays in the center channel of drop-down rest then clamp the TAC 15 on its side to measure the incline of the arrow vs. rail. With a Whisker Biscuit, there is no need to strap the arrow in place since arrow is already held in place.
For those with an Apple iPhone, there is a nice app that turns the iPhone (or iPad) into a Clinometer (accurate digital bubble level).
Hope this helps a few people get their TAC 15s either set perfectly for center shot or gives them a leap towards setting perfect center shot.