Having tested it a bit I can confirm it makes pretty much no difference where you put the BIZR. You don't need much either - a 2-pixel-thick layer around the outside will usually make the explosion last forever. Or you can replace about half the SING with BIZR and get the same effect (you don't actually need that much SING, since the particle limit prevents it all from working at full effectiveness)
It doesn't really matter where, personally I would put it in the middle - that way it won't spread around the screen too much, and your star will still have a (fairly) defined centre.
add some bizr where @Technomancer? and thnx for the tip
Add some BIZR and it will last longer (or, possibly, forever!)