M, it's not clear whether you are enquiring about fuel burn, timing or whether to start navigating from a known position or the o/h.
Fuel burn during taxi shouldn't amount to much, unless it's a big field and you get lost! I would be happy to calculate fuel burn from take off. I'm probably going to land with at least an hour left anyway.
Timing - start wherever you feel comfortable. eg o/h the field or a nearby landmark. I think you might be complicating a fairly straighforward matter.