I have tried too cut this large cove molding that looks to be on a 38-52 angle. When I go to cut my return piece it runs down hill. ( angle is not matching up. ) Any words of advice? thanks in advance.

You dont say where you're installing the molding, so Im assuming you're doing a crown.
If the return isnt staying level, then the molding isnt set to the proper projection and drop so your mitre is off
Dont assume the molding is any set angle- measure it by placing it in a "rafter" square and make the back cuts fit tight to the the body and tongue of the square (at least one of them, when the back of a molding isnt 90 degrees, and most aren't, I keep the wall edge tight). Write those measurements down. Top (horizontal) = Projection ; Wall (vertical) = Drop
I mark where the molding is going on the wall in several places. I cut with the molding "upside down" on my 12" saw and mark my fence to the vertical measurement and make all my cuts to that mark. As long as you're always cutting to the same projection and drop, your moldings should stay level and square
Your current return going down hill? Push it up and measure the gap at the mitre and cut a new piece adding the gap distance to the bottom of the mitre. The profile may not match and some sanding may be required

Here are some tips on how to cut large cove molding:

Measure the molding carefully. Make sure you know the exact length and angle of the cuts you need to make.

Use the right tools. A miter saw is the best tool for cutting crown molding. If you don't have a miter saw, you can use a handsaw, but it will be more difficult to get accurate cuts.
Make sure the saw is set to the correct angle. The angle of the cut will depend on the type of molding you are using and the angle of the walls.

Make a test cut. Before you cut the actual molding, make a test cut on a piece of scrap wood. This will help you to get the hang of using the saw and to make sure you are making the correct cuts.

Cut the molding slowly and carefully. Don't rush the cuts, or you could make mistakes.

Use a miter saw guide. A miter saw guide will help you to make accurate cuts.

Clean up the cuts. After you have made the cuts, use a sanding block to smooth out the edges.

Here are some additional tips for cutting large cove molding:

Use a sharp saw blade. A sharp saw blade will make it easier to cut the molding and will help to prevent the saw from binding.

Measure twice, cut once. It is important to measure carefully before you make any cuts. This will help to prevent you from making mistakes.

Be patient. Cutting large cove molding can be a challenge, but it is important to be patient and to take your time. With a little practice, you will be able to make accurate cuts.

Might not be a 52/38 angle
Lots of cove molding are 45° angle
Is your miter saw big enough to cut the crown upside down against the fence?

I would saw the standard is to make cove 45/45. It's a symmetric molding. Whereas the majority of moldings are 38/52, because of the asymmetric profiles. Bed moldings would be the outlier.