If I understand correctly, the total tax is basically the sum of federal taxes and provincial taxes, so for Quebec this should give a marginal tax rate of 58.75% (33% federal and 25.75% provincial). So why do all the tax calculators give around 53%?
Under an agreement between the federal government and Quebec, most Quebec taxpayers get a federal tax abatement (reduction) of 16.5% of either basic federal tax (line 57 on Schedule 1) or minimum tax, whichever is greater. The tables of marginal tax rates for Quebec include the reduction for the abatement.
So 33% * (1-16.5%) + 25.75% = 53.305%