OK here goes, maybe this will help.
Round the piece from end to end.
On the end away from the chuck (this will be the bottom) turn a dovetail tennon, so the chuck has something to hold onto. Use your parting tool to identify where the bottom will eventually be, by making a shallow cut.
Now, cut the blank off at the height you want the piece to be.
Chuck up the tennoned end and adjust to run true.
Turn the cup into the end.
Sand and finish.
Part off, angling towards the center so that the outer edges sit flat.
That's the way I do it.
Bruce Shiverdecker - Retired Starving Artist ( No longer a Part timer at Woodcraft, Peoria, Il.)
"The great thing about turning is that all you have to do is remove what's not needed and you have something beautiful. Nature does the hard part!"