Just wanted to add that you should not worry too much about whats underneath, you will only see the top layer, however, if the lady already has an ombre dye to her hair I hope that you measured how far down it needed to go - if that makes sense?
With ladies that already have an ombre I only add the lighter colour underneath. With the custom ombres tbh they are more of a dip dye and I am personally not keen on the look (i had them myself, nothing against the hair, I just think it looks cheap) as it was too harsh. You will have to keep your sections perfect and ensure that your bonds are all the same distance from the scalp.
I hope the above makes sense. I have attached a pic of an ombre/dip dye that I did without custom hair, I just added 2 shades of blonde.