Without going into the maths side of things
(although on that front 3" extra sounds right, but if you cut 3.5" extra and you don't need the extra 1/2" you can always cut it off ) since you seem to have all the leaves etc already cut out, if you are doing the stem from bias strip, cut it a bit longer, wiggle it from end to end to correspond with the original design, but slightly stretch the curves (not the bias!) to fit your new length. Then place the leaves etc down but again slightly stretched out with a bit more space between them - or if you think the gaps a bit too big cut an extra one or two leaves and shuffle things up to fit them in.