To truly explain this I'll have to explain a bit about how Target Offsetting works in IMFD. If you already know this, please bear with me.
Target Intercept's default behavior is to "aim" for the center of the target body's mass. This provides the largest margin of error, soldiers and police are taught to aim for the target's "center of mass" for the same reason. This is a good thing when planning a transfer burn for an interplanetary trip. For a shorter flight like a Moon trip, or MCC's later in an interplanetary flight, it's not so desirable. After all, we don't want to collide with the target, we want a very near miss.
Target Offsetting allows us to adjust that "aiming point" away from the center of mass. This aiming point is defined in spherical coordinates, which is a bit confusing. Rad (Radius) determines the distance of the offset vector, and Lon (Longitude) and Lat (Latitude) determine the direction of the vector.
Adjusting the Rad alone is adequate for setting up a Free Return trajectory, but note that 8M is a starting value that will need to be adjusted some to attain the desired PeA. Note also that 8M is much more than the 1837k suggested by dbeachy - his method can provide a starting point but the actual Rad will typically be very different.
Keep in mind that Target Intercept uses a simplified two-body solution and has limited accuracy. IMFD's Map program uses a much more precise multi-body solution, so it's important to use Map (with Plan enabled) to "fine tune" the offset. Doing this allows us to adjust the offset to make up for the inaccuracies of Target Intercept's simple solution.
In order to use Offsetting effectively and efficiently, it's important to understand how the spherical coordinate system works. Rad is pretty simple - it's a distance (in meters). Lon and Lat are a bit more complicated.
Lon is referenced to the target bodies prograde direction (as determined by the reference frame selected - P30 (LVLH) or velocity frame. The IMFD manual explains the diference between these two modes. Lon can range between -179 and +180, and is measured in degrees. This "wraps around", so 180 +1 = -179. To put it simply, a Lon = 0 is "in front of", or "leading" the target, and a Lon = 180 is "behind", or "trailing" the body. A positive Lon is "outward" and a negative Lon is "inward". See the attached diagram for a visual explanation.
Lat (also in degrees) is referenced to the target's orbital plane, with zero being parallel to the target plane, +90 being perpendicular to the targets plane to the "north", and -90 being perpendicular to the "south".
So, if we want to enter a retrograde orbit around the Moon, a Lon of zero is fine. Same for entering a retrograde orbit around a planet farther from the Sun than we started from, or entering a prograde orbit around a planet closer to the Sun. To enter a prograde orbit around the Moon (or a outward planet) a Lon of 180 will work.
To use Offsetting efficiently, especially for interplanetary flights, it gets even more complicated. I'll use an Earth - Mars off-plane transfer for an example. Of course, Target Offsetting only works when the source is ourself, so you'll only be using offsetting for MCC's, after we've left Earth's SOI.
I'll assume we want a prograde orbit around Mars. First, I'll use Map with Plan enabled to check my current trajectory and find out which way I need to move the "aiming point" (this point is shown in Target Intercept, hitting the CNT button in Target Intercept will center the display on the "aiming point") and set the Lon to zero or 180, accordingly. I could, at this point, simply adjust the Rad to attain the desired PeA, but this will be inefficient. Doing this will most likely change my PeT, which means I'm changing my forward velocity (dVf in Burn View), and this is very inefficient during a MCC when I'm far from a gravity source.
So, after I've determined the general direction ("ahead", or "behind"), but BEFORE I increase the RAD above zero, I'll use Map with Plan DISABLED (trajectory shown in green) to find my current trajectory's PeT. I will then set the TIn in Target intercept to match the PeT shown in Map. Then I will enable Plan (trajectory shown in blue).
Map should be set with Ref = Sun, TGT = Mars, CNT = p-mars, and Plan, SOI, and Int should be enabled.
For a successful Off-Plane intercept, it is essential that the intercept occur at a node between our transfer plane and the target's orbital plane - this node is displayed as a blue box (and a blue dashed line if the node is outside our target's SOI)
We use Rad to adjust the PeA, Lon to adjust the PeT, and Lat to adjust the position of the node. However, adjusting one value will change the others as well. For instance, adjusting the Rad will change not just our PeA, but will also affect our PeT and move the node.
It's often easiest to adjust one thing first, then two things, then all three. For instance, I'll adjust the Rad to get my PeA (as shown in Map) about where I want it. Then I'll alternate between adjusting the Rad and the Lon until both PeA and PeT are about right, then I'll alternate adjusting the Rad, Lon, and Lat until I've got the PeA, PeT and Node where I want. It's a very painstaking process, and you will need to redo the offset for every MCC you make.
This will be explained more completely (with screenshots and flight recordings) in the IMFD Full Manual V2, in the Target Offsetting and Advanced Concepts sections. I'm way past my "deadline" on this, I know. I swear to the probe I'll have my part done in time for a Christmas release.
