Blu-Ray disk capacities really?

[Tom Horsley <horsley1953@gmail.com>]

I'm trying to figure out how to stash data files on my new
Blu-Ray writer under linux, and I'm using up media faster
than I'm learning anything :-).

Supposedly, I can write 25025314816 bytes to a single sided
BD-R, and if I collect a list of files almost that big and
run them through mkisofs -print-size, it tells me the iso
image would be 12066268 sectors, or 24711716864 bytes, which
by golly should fit.

But if I try to burn that same collection using k3b, it
gets about 98% through and tells me it ran out of space.

If I run dvd+rw-mediainfo on another blank media, I see
this totally confusing information:

READ FORMAT CAPACITIES:
 unformatted:           12219392*2048=25025314816
 00h(3000):             11826176*2048=24220008448
 32h(3000):             11826176*2048=24220008448
 32h(25000):            7369728*2048=15093202944
 32h(1000):             12088320*2048=24756879360

That 24220008448 number is awfully close to 98% of
24711716864, so is that the real capacity of a Blu-Ray
for storing data, or is the default write technique
k3b is using not the one I want? What the heck do those
different format capacities mean?

Thanks for anything you can do to un-confuse me :-).
(k3b, by the way is absolutely hopeless at estimating
sizes - it tells me there is 2.1 TB of free space
on the media).