You don’t need to subtract from the length (unless you want to add the elastic waistband)
You need cotton, superfine, size 1 + 3 mm hook
You will be making the Ternion stitch, but instead of dc you use treble crochet (yarn over x2, insert hook, yarn over, pull through stitch, yarn over, pull through 2 loops, yarn over, pull through 2 loops). Then 1 sc into the side of the treble crochet, chain 1 and skip 1 stitch.
You can make both panels identical or do as I did and make them different – on the first panel I made me tr stitch in the chain between each tr crochet, on the second panel I made the stitch into the tr crochet from the row before.
When making the tr into the tr you finish with 1 tr, 1 sc, 2 ch in the last stitch, turn and make the first tr in the first tr of the row. The 2 chains doesn’t count as a stitch.
Start each panel with a chainless foundation of treble crochet. Finish with 1 row of treble crochet (1 tr in the tr, 1 tr around the chain). To make the skirt straight, make sure to stretch the first and last row when you are finished.
When both panels are made slip stitch each side together but only over some of the stitches (1 ss through each tr crochet)