Ok, see if this meets your needs. I would caution you to lay out ALL your standard sized track around your benchwork in their assigned locations first. Then begin assembly with ALL the standard pieces for the left loop section. Once that is done, you can A determine the exact length you want for section "A". Even though it says 23", you might find that 22" or 24" works better for positioning the loop and the straight track along the top. Don't worry about deviating from what I've got. Since you'll be custom cutting pieces for the main straight sections, they'll all key off one another.
For example:
"A" determines how close the left loop will be to the bottom edge of the benchwork. It also determines how close the top straight section will be to the back wall. Therefore, you should cut it to size based on where you want that loop to be on that leg of the horseshoe, not specifically to what my measurement came out to be.
"B" determines where the right loop will be positioned with regard to the wall on the right.
"C" determines where the right loop will be positioned with regard to the edge of the benchwork.
"D" determines how close the inner siding will be to the back track.
"E" determines the position of the outer siding at the horseshoe.
"F" determines how the left side of the horseshoe will line up with the right. The tracks attached to "F" have to go straight across to "G".
"G" finishes that section.
"H" determines how the left side of the inner siding will line up with the right. The tracks attached to "H" have to go straight across to "J".
"J" finishes that section.
If you followed all that, you'll see that my measurements are just a guide and you'll have to make your own measurements as you assemble things. This is exactly the approach to assembly I'm going to eventually use. I'll use standard track for the curves, but all my straights will be use flextrack cut to a custom length unless a standard straight piece will fit like at "H".
Oh, and don't forget those 4 filler pieces that should have come with your turnouts.