The P3.5 indstruction sheet gives instructions for mixing 1, 2, or 5 litre batches.
Stir after each addition.
Developer: 400ml water + 300ml part A + 100ml part B + 100 ml part C + water to 1 litre.
Bleach: 400ml water +200ml part A + 100ml part B + 100ml part C + water to 1 litre.
Fix: 400ml wter + 500ml fixer + water to 1 litre.
My only concern is keeping qualities of the partial containers of stock solution.
I will probably break it down to the smaller bottles from my last P30 kits.
I haven't used my P3.5 kits yet - I am finishing up the P30 I was lucky enough to get from Darkroom Deals.