I think this answer is going to come down to specific answerS for each type of part.
My RT has a Morse taper in it's center. I purchased an unhardened MT adapter for a large size Jacob's chuck so that it would have a large tapered projection (a Jacob's taper) at the front end. It just so happens that my lathe's spindle also has the same MT as the RT so I slapped that MT adapter in the lathe and turned that large center projection down to a known diameter which was not tapered. Then I drilled out the center and bored it to exactly 0.4000" diameter which is exactly twice the diameter of several of the edge finders that I have (0.2000"). When that MT plug is in my RT I can center the RT under the quill with an edge finder with a few back and forth, X and Y motions.
And the OD of that plug, being known and exact, can be used to center or position a work piece on the table. How that is done depends on the actual work piece. There must be some features on that piece that you need to reference when you determine a center point.
In the case of a radius between two edges, then those edges would be the features to use, along with the radius of the desired arc between them. A way to measure that radius, plus or minus the radius of my center plug, would be used to space the part from that center plug.
Milling two flat sides at a specific angle to each other and with no need for locating a center point would be even easier. Simply mount the part and mill one surface, using whatever features it must relate to. Then turn the RT by the needed angle and mill the second surface, using the mill's X and Y movements and again referencing it to whatever features it also must relate to.
Milling two sides that must meet at a point that is not on the part is a bit more difficult. The part would need to be aligned to the center of the RT, using whatever features needed to do so. Holes, edges, reference points, etc. There must be some features that already exist on the part to do this. Otherwise, the first edge could just be milled at random and then the second one could be cut as described in the paragraph above. Once the part is aligned with it's "center" at the center of the RT, then the table would need to be turned until the first cut is angular aligned, again with relation to some features on the part. Then, once the center is established and the angle of the first edge is set, the X and Y movements of the table can be used to position the side of the cutter along the proper line. My MT plug in the center of the table would be used to find that center of both the RT and the part and then appropriate Y movement would be used to establish this position of the cutting tool's OD. And the first edge would be cut. Then, the RT can be rotated by the needed angle between the two edges and the Y table movement would be used to move by the diameter of my plug PLUS the diameter of the cutter. The second edge would then be cut using the opposite side of the cutter.
I know all this depends on the "known" features of the part. But, not knowing those features, it is not possible to be more specific about their use in any of these procedures. If you have problems with a specific part, then post photos and/or drawings of that part and I/we can tackle it.
