Hi Gator Bait.

When using downriggers one of the critical factors is blowback. Blowback is how high your ball rises in the water. This is controlled by weight of the ball and your speed. The lighter the ball or the faster you troll the more the ball rises up in the water.
Because of blowback you can't use the precision trolling app exactly, you have to adjust for your particular blowback. In general you will have to set the rigger deeper than what the app says.

The easiest and best way it to use a fish hawk that tells you the exact depth (and speed) of your ball and hence bait. But like everything else they can be pricey. Before we had fish hawks we would let the rigger down when at trolling speed until it just bumped bottom. The difference between the rigger counter and your depth sounder is your blowback. Once you know this you are good to dial the rigger precisely to the depth you want for any depth.

It is not as bad as it sounds because after a few times you will know your boat/rigger blowback and won't need to check anymore.