Ilford Multigrade Mixing Questions

For storage and shelf life reasons, is it better to mix batches of 100ml Multigrade with 900ml of water, or should I mix all 500ml with 4L of water and use 1L of solution at a time?

I mix it when I'm going to use it. I keep the stock developer in empty 500ml bottled water bottles. Squeeze the air out and it will not oxidise. I would be a bit leary keeping a working solution this way though...

Once you mix the stock with water, this working solution life is much shorter. Just mix what you need for immediate use.
I mix up enough working solution needed for the developer tray (64oz in an 11x14 tray works for me) and it'll usually last a few days before I dump it out and make a fresh batch.
1:9 seems to last a bit longer than 1:14.