You will never get it to look level because there will always be that 1/8"
difference between the areas. You should have pulled down the 3/8" drywall
back to a solid divider and installed all 1/2" material.

That said, I have "adjusted" some "bumps" in a ceiling where a beam crossed
an area a little lower than the ceiling joists. This took a lot of compound
even when I used a slightly dried pail to fill the deepest area to reduce
shrinkage and cracking. It took probably 8 coats to keep replacing the
shrinkage, fill cracks and to smooth bumps. I probably sanded off as much as
I left in place to keep it level. It did not want to level very well because
I was using a 4 foot straightedge to scrape the compound flat and to spread
the fill far enough to avoid a shadow line. It worked for my needs in this
particular area. Your results may differ because it is an open room and
windows may emphasise the difference between the levels. You can either
change the thin drywall or do something similar to what I did, hope the
results are acceptable to you.

"Mikepier" wrote in message
ps.com...
I recently gutted out my kitchen and took down a wall next to a dining
room, where there was 3/8" sheetrock on the ceiling. I did not want to
put 3/8" sheetrock on the ceiling in the new kitchen due to concerns
on sagging, and plus I wanted to use 12 foot boards, so 1/2" sheetrock
was the choice. I tried my best to spackle where the 2 meet ( 4 times
already) it still seems its noticeable, although each time I do it it
seems to get better. What is the best way to spackle the joints?