In order to get the most performance from your bands (tubular or flatbands) you must first know your draw length.  Once you know your draw length, you may then trim your bands to your active band length.
Your active band length is the length of the bands or tubes from where they attach to the slingshot to the pouch.  This length is determined by dividing your draw length by 5.  Example:  If you determine that your draw length is 30″ your active band length is 6″.  30÷5=6 or 6″.   This is the sweet spot for most elastics.  If you shorten beyond a factor of 5, you will see an increase in velocity at the expense of shortened bandlife.  Likewise, if you increase the active band length, you will se a drop in speed with an increase in band life.