The body reviewing the workings of Scottish devolution after 10 years is due to take evidence in London.
The Calman Commission was set up in March to look at how things have changed since the Scots voted in favour of creating their own parliament.
It will consider the Barnett formula, used to allocate public spending across the UK based on population share.
Sir Kenneth Calman has said all the evidence received has called for the formula to be based on needs instead.
The Barnett formula was devised in the late 1970s to automatically apply a proportionate share of any change in comparable spending programmes in England to Scotland.
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Lord Barnett, who created the formula when he was Labour's Treasury chief secretary, has said the formula was never intended as a long-term measure.
Sir Kenneth says the question of how Scotland is funded is the '' big issue '' facing devolution.
He said he and his commissioners had not yet come to any final decisions on whether the Barnett formula should be changed.
In a BBC Scotland interview ahead of taking evidence at Westminster, Sir Kenneth said the Barnett formula '' is a remarkably simple formula to operate, it doesn't cost much so there are some very positive bits about it and any change would need a lot of debate and discussion''.
The Calman Commission is due to publish its final report in early 2009 but there are serious doubts that the government in Westminster would be prepared to overhaul the present funding mechanism.
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