[AngronIsAngry]

Also a "Welcome to SubSim" from me.

Within the unmodded stock game there is only 2 methods of inputing range into TDC:
- Stadimeter in Perisocpes and TBT (either by matching-the-pictures or pulling the triangle on the dial)
- Active Sonar Readings

There is a catch to every method
Aiming for the waterline and matching the mirroed waterline with the ships topmost point, only works precisely from close up, say 800 to 1400 feet.
Beyond that, the mistakes become worse exponentially.

If you use the triangle to manually input the range, you need to identify the target with the recognition manual (key-shortcut n) to get rid of the ~1500ft limitations. From then on, it is a matter of a steady hand and coping with a +/- 50ft error.

Also the sonar readouts have problems. Unless you are running on easy settings, any readouts on large ranges should be considered "somewhere about"-range. Good enough for planning approaches in pre radar time or post radar damage.
Upclose you should still take multiple readings. The error won't be that big any more, but you will have to consider it.
Also keep in mind that every ping is a big loud scream of "I am HERE!". So be careful with it, when escorts are around.