(Sandy's mixing instructions have caused some confusion. I think I even saw a Formulary pdf that said erroneously "add water to 100mL". Maybe it's been corrected since then.)
Just for the record, I don't have a thing to do with the Formulary kit so don't blame me for whatever mistakes they may have made in packaging or in the instructions. If they were paying me for the use of the formula I might feel some obligation to make sure they have it right, but as things stand that is their responsibility .
In any event doubling the amount of water and mixing the working solution 1:2:100 (or 2:4:100) should work fine, but you may need to adjust develoment times down slightly because mixing this way will result in slightly more carbonate per liter of working solution, making the developer a tad bit more energetic, or fast working.