This is what I did:
Create your report as usual, with detail and subtotals and such.
Then, name each textbox uniquely in each subtotal:TextboxSub1SalesUnits (for the first level of subtotals), so on and so forth. Then, on the expresion for the detail line, use this formula to calculate the percent to total:
=(Fields!SalesUnits.Value / ReportItems!TextBoxSub1SalesUnits.Value)*100

Then, on the subtotal 1 level type:
=(ReportItems!TextBoxSub1SalesUnits.Value/ReportItems!TextBoxSub2SalesUnits.Value)*100

If you want, you can make textboxes for the totals by warehouse (and hide them if you want to), and still make reference in any calculation.
It's a pain to create all those textboxes, name them and make the formulas, but it's the only way I know so far.
Hope it works!