Excluding the distance between between the track centers and the table edge the minimum width is the curve diameter. The minimum length for the loop to return to parallel is 1.5 times the curve diameter. If the loop is not going to close with itself but exit parallel to the entry then the required width increases by the desired separation.
For your O99 reverse loop with a 4.5" track spacing between the entry and exit tracks is 99" + 4.5" or 103.5". The length would be 1.5 * 99" or 148.5".
Here are some possible configurations of reverse loops. These are done in O54 but they illustrate the idea. The first example is the minimum as described above. By inserting 4.5" straight in the right side the entry and exit will be separate but parallel.
Jan
