[Nonfractal]

Thanks for the reply.

The reason I asked is that I didn't know where to start on a current Slot.it 12/27 anglewinder build, so I simply asked the people selling the stuff, who said...

"...use the 6.75mm 12t pinion with the [16mm] SIGA27pl". 

So looking at your list, it sounds like they like their meshing nice and tight.

Jeremy,
Slot it angle winder pod. Long ratios.
It was suggested to you that you use: 6.75 (12T) and Si-Ga27pl 16mm (27T) Ratio 2.25
6.75+16 = 22.75 (sum of the diameters) and is 0.75mm over spec.
Tight? Certainly. Nice? Not at all !  

11T/25T = 2.27 is very close to your desired ratio and can be achieved with stock Slot.IT gears.
6.75mm 11T pinion Pi-6711o
15mm 25T crown GA-1525PL
(21.75mm total. 0.25mm too short diametrically)

21.75mm comes up to 22mm with an 0.5mm axle spacer on the inside of the crown.
(0.25/2 x 3.6 =0.45)
The 15mm crowns are also available in 24 and 26 so that also gives you flexibility ability to lengthen or shorten the gearing a little without having to pull the pinion.
There are other options but I think this will best meet your needs.
Hope this helps