Gee IF I had a baby I might be able to tell you BUT my baby turned 19 last week so I'll send this to the top:)

Barb, had you though of putting it on the baby and just marking it? Myself - I would take the largest part of the onesie at the bottom and make that the center - but I don't have one - to work with here. *4U

this is what I do with that sort of design placement.. I go 1/2 inch up from the highest part of the curve of the leg .. mark the center ..

Barb the size would play into that factor, I', sitting here with a onesie n my hand size 0-3mos. & the design to go on the butt would around 10 inches down. If the baby isn't available, try it on a doll or teddy or something like that. *

If i could get out of bed I would check for you, but failing that, I would take the onesie and fold it in half from top to bottom to find the (waistline) centre.
Then I would measure the distance between the waistline and down to halfway down the leghole (toward the buttonholes) this should give you the "bottom" placement.
Clear as mud!! hope this helps, hugs n roses, Meganne