I made another cestros (or kestros) sling last night and took a few photos and measurements.
I used thick jute twine and a section of a broken arrow shaft:

The peg is a little under 2" long. I whittled a point on one end and a shallow notch close to the other end:

I then took two lengths of cord. Each was three times hand-to-opposite shoulder (a total of about 3 metres or ten feet)

I reverse twisted these together to make a stouter cord:

From the release knot I measured the length that I wanted to suspend the dart at (hand to centre of chest, 32") and tied the Alpine Butterfly knot there. I worked the knot before tightening it so that there was enough loop to go around the shaft of a dart with plenty of room to peg it up. The length of the loop ended up at 4". I then tied in the peg using a Constrictor knot around the shallow notch. The distance from the loop to the peg was also 4".

Lastly, I tied a retention loop at a distance that left some slack in the retention cord when a dart was pegged into the loop and held by the release cord. This came out at 38" from the peg to the top of the fingerloop.

Summary:
Release knot to dart loop knot 32"
Dart loop 4" long
Dart loop to peg 4"
Peg to tip of retention loop 38"