Thanks I know how to use them. So knowing the atmosphere boundary height is enough.
Knowing the atmosphere density at a certain altitude is better.
Depending on which spacecraft you are using you want to holdalt at an altitude that gives you a reasonable deceleration rate without too high G's. Maybe 2.5 to 3 G's tops to simulate a realistic reentry for human comfort.
Research the maximum dynamic pressure for the spacecraft you are using and never exceed this.
Titan has very high atmospheric density so you have to be careful not to set you perigee too low into the atmosphere as dynamic pressure will build up fast. On the other hand, Mars has low density so you have to dip deeper into the atmo. lest you skip out by not slowing down enough. About 25km to 35 km.