The figure contrasts with the party's own statement that its plans would see the Scottish Parliament raise 40% of what it spends.
Labour's Devolution Commission has published its final report on the further transfer of powers to Holyrood in the event of a No vote in the independence referendum.
It proposes giving MSPs the power to vary tax rates by up to 15p in the pound, giving Scotland control of three quarters of the 20p basic rate of income tax.
MSPs would also have the powers to increase the higher rates of income tax, giving it the ability to tax richer citizens at a higher level.
Reform Scotland, which set up the Devo Plus group, has now published its analysis of the Labour plans. The organisation applied the proposals to the latest economic figures from the Government Expenditure and Revenue Scotland.
Its findings show that Holyrood will raise in tax only 26.18% of what it spends, marginally more than the 22.49% which will result from the Scotland Act - the most recent transfer of powers.
Holyrood will raise in tax 20.07% of total tax raised in Scotland, only marginally more than the 16.31% which will result from the Scotland Act.
Reform Scotland says its figures take into account the additional spending powers included in the Devolution Commission report - £1.7 billion in housing benefit and £500 million in attendance allowance, and the additional tax power in the form of an additional 5p of income tax, which the report estimates at £2 billion.
Ben Thomson, chairman of Reform Scotland and Devo Plus, said: "Labour's proposals increase the Scottish Parliament's tax-raising powers by less than 5%, and represent only 26% of Scottish Government expenditure, which falls well short of the 40% they are claiming.
"The report is clearly motivated more by short-term referendum politics than a real desire for significant further devolution."
A Scottish Labour spokesman said: "As confirmed by the Scottish Parliament Information Centre, our devolution proposals will bring the amount of tax raised in Scotland to around 40%."