One can fit 4,800,000,000 on a regular DVD. That leaves me with about 125,000,000 left to spare
That is a wrong assumption. Just because the media claims to have 4.7 GB of space, that doesn't mean it is exempt from the 2's complement math that computer use to calculate actual space. (1000 bytes is actually 1024 bytes of space taken up)
You really have barely 4.37 GB of total space to write to on a DVD-5.
My question is HOW MUCH extra space does DVD Studio Pro add?
None - Really - DVD SP just muxes together the assets you place into it.
I came up with a total of 4,575,000,000
Does that include your audio as ac3 Dolby? If not, you can save several MB of space compared to .aiff.