I have the Rollei/Digibase C41 kit, which makes 500ml of developer. I don't presoak, so with each roll (I'm about seven in), I'm losing some volume. I now have about 450ml of working solution -- not quite enough to cover a 120 roll. I figure I need to add between 25 and 50ml of water. Can someone help me gauge by how much I'll need to compensate in development time? Or is this just a bad idea?