So, actually the needle arrangement on my main bed is 7 working, 5 not and with half pitch (H-5), the rib bed is 6 working, 6 not - it makes sense to me.
Now because of the 7/5 on the main bed, and to have 3or 4 needles in work at sides on main bed for seaming, I like to even it out by beginning with an even-number-stitch rib at the very centre (6 sts, 3-0-3 n’s) so the same needle numbers will be in work at each side - the ribs will then match at the shoulder without having to worry about using an uneven number of needles and then remembering to flip the needle arrangement used on the back for the front.
This way, I think it is easier to check that you have the correct needle arrangement and not one skinny or one wide rib somewhere randomly - putting it in the centre (not shown in my swatch) will not be noticeable and for the front where the piece is divided, it gives you the same number, even stitches for each centre front edge.
To lay out the needle arrangement, I begin by bringing 3-0-3 n’s on MB to work. Then, working to right, 5 out, 7 in, 5 out, 7 in to edge - the width of your garment may be decided by where you can end with 4 in work - in my case, I want the width at the bottom to be 30 to 32 cm, so I’m either stopping at #84 or pulling another 12 n’s (96) to end with the 4 in work again. Another way to do this would be to simply add the 4 needles in work at the outside edge for the width you want, regardless of the needle arrangement but I like the look of my way because I am shaping the sides - A-line-ish.
I want to have a no-hem look, just use the manual wrap cast-on, but over this needle arrangement, it could be tricky, so I’m going to cast on with waste yarn (WY) and ravel cord before using the main yarn for the wrap - this way, I can hang the comb and weights in the waste yarn without worrying that the wire/comb will damage the yarn in the cast-on row, making sure everything is working before getting to the real deal. I am not going to hand wrap the WY (too much unnecessary work) and if I cast on full needle rib, there’s too much transferring up and down and making up new stitches and then worrying about the needle arrangement being correct again - Here’s the trick - after setting up my needle arrangement, I rack the rib bed (RB) until the needles on RB line up with the in-work ones on main bed (MB), to get a zigzag row....H-11 does it. Now bring up one needle on the RB at the left of each group of in-work needles and at each end look at what needs to be brought into work for the ends. WY, T5/5, K1R. Hang the comb and 1 weight. K1R. (this sounds ugly and clacky while you’re doing it because of the wide space of needles out of work - not to panic!) Rack one space to H-10. K1R. Begin getting rid of the extra unwanted needles at the ends by transferring the sts back up or down after each row. By the time you’re back to H5, it begins to sound better and you should just need to transfer that one extra ribber needle of each group to be at the correct arrangement. Check it a second time, add extra weights to balance everything out and end with the carriage at left. Knit the ravel cord ending at right. Bring all the n’s out, manual wrap with the main yarn and Bob’s your Uncle!!
If you want a review of this cast-on and decrease method, see Ribber Rules 5 & ‘Just Ribbing’ in in KNITWORDS No 50.