420 divided by 23 litres comes out at around 18 (which is offensively low for a lot of people on this site). I'll be hacking the kit up to about 30, while TBN will be adding nothing but time (and a kilo of sugar).
I know that Sbillings will be producing a blondish ale at about 30 IBU, using Hallertau so we should set the IBU range between 18-30. I suppose the SRM should be less than 10 as well.
Beyond that, I reckon there should be some lattitude. Yes this will be an interesting experiment, but its supposed to be fun as well!