Rib for cuffs, hem bands and necklines is cut shorter than the opening so it grips the body. The basic rule is 65–85% of the opening length, depending on the rib's stretch and the look you want. For a t-shirt neckband, rib is around 80–85% of the neckline; for hoodie cuffs and hem bands, 70–80%. Elastane in the rib allows a lower ratio. Test on one piece, since each fabric behaves differently.
What rib ratio should I use for cuffs and neckbands?

ribneckbandsewing
Weight, width & measurementWhat is the difference between net and gross weight on fabric rolls?
Net weight is the weight of the fabric alone; gross weight also includes the packaging: the film, the cardboard core the fabric is wound on and possibly a bag or…
Weight, width & measurementWhy do two rolls of the “same” fabric weigh differently?
Because rolls are not identical: they can have different lengths, slightly different widths, GSM values within tolerance and different moisture content.
Weight, width & measurementHow does moisture affect fabric weight?
Fibres absorb moisture from the air, so the weight of a fabric changes with its environment.