[winger]

Thanks Kev.

The Arista calls for 1/9oz, and I've been mixing it 1/18oz, and doubling the development times. I haven't seen a problem, but thanks for the exhaustion part. I didn't know that, so I'll have to watch it.

You mentioned 1:1 and 1:2 above. Not sure why? You aren't using 1:1 or 1:2.
Yes, when you make it more dilute (same amount of developer in more water), you need more time. However, I'm not sure why you'd decide to do this because you don't print much. Do you mean not often or not many prints when you do it? If you use it more dilute but the same total volume, that means it'll take you longer to use up a bottle, which means it's more likely to go bad before you finish it.
And I think it's much easier to do the math if you use ml and L rather than oz. An 8x10 tray works well with 1 L of working solution. 100ml developer and 900ml of water gives you that.