This is the jig I use to place the bricks in just the right spot to get a nice even dome.
It's made from 10mm steel rod, with a piece of 40x40 angle welded onto it at right angles.
The length of the jig will determine the size of your oven. It determines the radius of your dome.
For this oven, I'm using a jig 500mm long. This will give me an oven with a diameter of 1 meter.
This is a bit smaller than the 1100mm diameter of the first oven I built, but I still think it'll be big enough.
The next job is to find the centre of the dome and mark it.
I've set my dome back a bit from the front in order to provide room for the smoke chamber and work area.
Once I got my mark, I stuck down a 10mm washer to stabilise the jig as it pivots.
Footnote: The washers didn't last into the second day of laying. I ended up drilling a 10mm hole in the brick (to about 4mm deep). This worked much better.