the reason for the excess with the long arm is to have fabric to attach to the leads- top & bottom & hold with the clamps on the sides and to allow for the (take up) caused by the quilting/stitching process- it also helps if everything is not totally square so you don't run out of backing/batting as the quilt is rolled.
using a domestic machine is a complete different process so the needs are different.
make sure you baste well-
if doing embroideries choose ones that are quite (open) a heavy embroidery will draw the fabric up ALOT...the excess need will be totally dependant on how close you plan to embroider--the process of decorative stitching 'draws up' the fabric- the bulk of the batting also causes the fabric to draw up some- if you've ever done embroidery/applique you will know what i mean- so excess fabric/batting is needed- and can be trimmed off later. quite often i notice on applique patterns they recommend cutting your backgrounds 1" larger - do your applique- then trim block to the correct size- it does depend on the amount of embroidery you plan to do-
if you plan to do regular quilting (not embroideries) it depends on the density of the quilting- some batts need to be quilted every 2-4" which is quite dense and will 'draw up' more than a batting that is quilted every 10"...lots of variables. it is better to have too much & trim when completed- than to run out before reaching the end & having to figure out how to add batting/backing to the bottom in order to finish.