Copying a large plaster tile by latex moulding it; is my plan correct? I have a large rectangular plaster tile with an interesting relief pattern of bumps, dips and waves, but it's broken and I need to make a copy of it. It's not a bad break, 3 clean snaps making 4 pieces that go back together easily
My plan is to:

Make a wooden frame around it, a few inches higher than the highest relief and lined with a strong plastic membrane
paint it with several layers of latex
talc coat and peel this latex off and then lay it back on so it's loosely on the original
mix up some normal drywall plastering plaster (I've got loads kicking around, too old for plastering the wall with) or sand/cement mortar mix
pour this into the mould on top of the latex and let set to form a supportive reverse mould
lift off the plaster/latex reverse mould and separate the latex/plaster from each other
flip the whole thing over and reassemble the reverse+latex+frame and set about filling it with plaster of Paris and maybe some gauze/mesh/glass fiber weave that I have to add a bit of reinforcement
separate the newly cast panel from the mould, peel the latex off it and repeat if I need more

Have I thought of everything? Will the latex separate easily from drywall plaster/plaster of Paris? Do I need a release agent? Will my chosen materials suffer any shrinkage or warping (they don't seem to when left in a bucket after a plastering job; it'd be a lot easier to clean if they did!)
You're on the right path; the only things I would change in your plan would be not to remove the latex mould (often known as a print mould) before adding the supportive plaster mould (known as an overmould or mother mould). Shawn Thorsson's blog has a good tutorial on one piece moulds - he works in silicone rubber and plastic, but the principles are the same.

Plaster* and latex don't stick together much, so you shouldn't have any issues without applying talc or another release agent as long as you let the plaster dry for long enough, but there's no harm in applying one - the Hirst Arts website has a good guide to them.

Both latex and plaster do shrink a little, so consider applying multiple coats of latex to get the correct thickness when building the mould. Since you're doing a one piece mould, you can deal with the plaster shrinkage by over filling it and then scraping off the excess. Once again, Hirst Arts has a good tutorial on casting one piece plaster moulds.

*Admittedly my casting experience is solely with dental stone, which may be less 'sticky' than drywall plaster. Your mileage may vary.