Your question needs to be more specific as to your needs.
On a typical reverse loop you run straight, then have curves to change direction 180 degrees. After that you have to decide how much turning you want to do to close the diameter you've used to go the 180.
If you only want to turn one more sectino of O72, then your loop will be almost 20 feet long to re-connect with the starting turnout.
If you want the tightest by going 90 degrees more followed by 90 degrees back (counting the turnout's curve), your whole reverse loop will be about 9 feet long.
And then there are about 4 intermediate options.
The "tight" loop needs 5 of 10" straights, 1 of 5", and 2 of 1 3/8", plus the two required 1 3/8" for the turnout itself (and 15 of the O72 curves). One of the 1 3/8" goes after the first curve off the straight (to match the required turnout peice). You could connect up without that one piece as you have so many sections to spread out the "scrunch" without it.
--Joe.
