A 4'x8' sheet of sintra was more than enough
for all my Jango armor pieces...including my
gauntlets. I didn't make my arena jet pack from
sintra so I'm not sure how much you would need
if you are going to make one that way. I only
used a little over half the sheet though for all
the armor.
As far as the amount you want to use...if you make
no mistakes, and space the cut-outs close together,
you should be fine. I, however, always like to have
extra on hand in case of mishaps (which happen more
often than not in my case!).