The broadband speeds of Malcolm Turnbull’s FTTN plan depend on a number of factors, including the quality of the copper, its diameter and how far it runs for.

Detractors, like Kieran Cummings (who tweets under the handle @sortius) claim the plan fails on all these counts. On his blog he says it’ll take millions of cabinets to achieve the proposed speed, but he could be prone to exaggeration.

So, how did Malcolm Turnbull arrive at his figure of 50,000 nodes and how far the run is from the node to the home? On this week’s CrossTalk he has an answer that we hadn’t been able to find in the public domain – and we asked a lot of people.

But it’s only an average. What about those houses that fit within a node area but have a copper run length way above the average? Paul Brooks from Layer 10 Advisory says there are many cases where some houses are set a long way back from the road, for example.

Turnbull’s average distance from existing pillars give a ball-park indication of how practical the Coalition’s broadband plan is, but could the cost of outlying properties push the price up higher?

Brian Beckor from goemapping specialists Callpoint Spatial, says only Telstra has the data needed to figure that out – and reckons they’ve probably already been through the exercise.

In which case, wouldn’t a reality check help Malcolm sell his plan and broaden the public debate ahead of an election?