I used a steel rule and spaced the cog so it was at the same distance from the centreline as the chainring.

I used my mad math skillz 😈
* my rear hub width is 135mm
* shimano middle ring chainline is 50mm
* did the math (135/2)-50 = 17.5
* measured 17.5mm from the dropout to cog center
works fine

Put chain set and ring on.
Put sprocket on free hub body no spacers
Put chain on.
Spin the cranks and the sprocket will align itself.
Work out how many spacers you need each side of the perfectly centred sprocket.
Fit spacers and lock ring.
Go for a ride.

1) Random number of spacers on the left, sprocket, spacers on right untill lockring tightens.

2) Does it look about right

Yes - ride
no - repeat 1

Straight edge. Place it against the outside of the chainring and see where it ends up at the back. Rotate the chainrings to a couple of different positions to check that it's not bent. This method is directly measuring the thing that matters - the angle between the chain and chainring. That said, this method only confirmed what I could already see by eye...

Measuring and calculating the distance from the centre line depends on your frame being true. Mine was off by a couple of mm.

[b]Spoons [/b]got it.

Stick bike in stand, rotate drivechain, engage calibrated eye, repeat.

*find hill, slip chain, get off, allen key out, fix*

There are easier ways, i'm sure.

Customer's bike: use a chain line gauge, and move spacers around until it's exactly straight.

My bike: randomly put it together because it really doesn't matter in the slightest 😉

Slightly off topic but y'know what I realised while making a Singleator today?

That device removes the ballache of spacing.

slideyslidey.

I do it by eye and run a proper unramped ring and SS chain