Probably will be some variation between jurisdictions....
Ignoring the potential value of a CAD layout analysis, the traditional method involves physical taxi trials for 90deg/180deg turns. A suitable clearance allowance is determined from the high strength side and end alignments and the test numbers normalised to account for whatever "standard" clearance is adopted.
It would not normally be necessary to complete the turn and line up on the centreline as that is not the aim of the exercise .. rather to part complete the turn to a point from which a sensible and repeatable takeoff roll can be commenced.
What final figures are assigned to the particular takeoff calculations will depend on the relevant Authority but will be from the runway end (real .. or notional in the case of an intersection departure) either to the mains or nosewheel.