Hey! And welcome to Subsim!!
Learn to get familiar and love your TDC!! It takes all of the math out of a solution, well almost all of it.
At the bottom there should be two dials you cant adjust, if your TDC is not in manual mode (It shouldn't be after you enter your Speed, AOB, and stuff) (OR If you use the Watch Officer Window to set it, either way it works) then the dials at the bottom should turn as you turn your scope, this is called your Gyro Angle. The one on the left is degrees the one on the right is increments of degrees, basically you want those dials to have both of their dial arrows pointing upward, indicating a Gyro Angle of 0. This means that when your scope is there, whatever crosses the crosshairs of your periscope, will be hit right where those crosshairs are aiming. Hope that clears things up!
Any more questions, feel free to ask!!