Nets can have "impedance_rule". A trick used by many fabricators is to set a line width to something easy to pick up on, for example, set a line to 4.876 mils. Then tell them that "all 4.876 mil lines on layer xxx shall have an impedance of 50+/-5 ohms. You can check if "impedance_rule" gets forwarded to ODB++. I personally do not use it. However, with Allegro, why not go ahead and set up the stackup with the correct material values and set the impedance in constraint manager to 50 ohms (or 100 ohm diff) and route away? The Allegro solver is very accurate and uses the same closed-form equations as the fab houses. What differs is actual resin content and thickness of the materials. Speak with the fabricator up-front and get this data and enter it into your material database and let Allegro do its job.
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