The method used in Home Direct was specifically designed to be the simplest way to find the GET without doing any serious math. It is not the best, or most accurate way.
For trips TO the moon, TIn (and the GET below it) is used to help adjust the Time Of Flight. This is done to make the trip a bit faster (about 3 days compared to 5 for a Hohman transfer), and this extra velocity is helps when performing a free return trajectory. You just adjust the GET to establish a Time Of Flight of around 75 hours.
Going from the Moon to Earth, the GET for TIn (or arrival time) is important if you plan on a direct re-entry. Since the Earth is rotating, arriving at Earth at the right place for re-entry requires the extra effort in timing - The Earth Pe must be at a position (longitude and latitude) that is in range of the target. "In range" will vary from vessel to vessel. Base Approach, in re-entry mode) can be used to figure this out, but it's rather time consuming. Set Base approach for the target base (or set the Longitude and Latitude manually), then adjust the ReA (re-entry angle) and Ant to suit the vessel and re-entry profile you will be using. The default values are appropriate for Apollo style capsules. For a DG type vessel, set the ReA to 1 - 1.5 degrees, and the Ant around 90 - 120. Ant (anticipation) is how far the vessel will travel, in degrees< after it crosses the re-entry interface (default is 120k, this is fine for Earth) and when it lands. For gliders, it's best to set this a bit high - it's easy enough to extend the glide.
In Base Approach, set the source to Moon (not self) and add som HInt until a solution is found. Then you can increase the TEj (and will likely have to further adjust the HInt when the solution disappears) until you find a suitabley low DV. Surface Launch and Orbit Eject can be set to get info from Base Approach, and are used to launch and eject, respectively. MCC's will be made with Base Approach.
It can take a LOT of fiddling to get a good solution this way, that's why I went with the "easy" method for Home Direct. That particular tutorial was oriented around the inverted re-entry technique, and the TEI proceedure was sort of an afterthought/bonus feature.
NOTE: whenever you set a GET or MJD value in IMFD, it is associated with a variable - usually TEj or TIn. The line directly above the GET is the variable it affects, so adjusting the GET under the TIn will change the TIn.
