Dump/Tape blocks question

[FreeBSD Questions]

I have a question about dump and tape blocks.  When I do backups, dump
tells me it uses a certain number of blocks for that particular
backup.  I explicitly set my block size to 65K when I do a dump.  When
I read the SCSI or hardware block location on the tape after the dump,
it's nowhere near the same number as what dump said it used.

How big is each block?  I'd like to know about where on the tape I am
when I do an 'mt rdhpos' command (but I don't know how to determine
the reported block size), and I'd like to be able to calculate how
much data was written for each backup based on the number of blocks
dumped (I assume these blocks are the size I specify on the dump
command line).  In fact, how can I calculate how many blocks my tape
can hold?  (i.e. What is the biggest block number an 'mt rdhpos' or
'mt rdspos' will report for my tape?)

Of course, an obvious solution to the last question is to write enough
data to fill the tape and then check the output of 'mt rdhpos'.  I'd
rather not spend the hours needed and the unneeded wear on the tape to
figure that out.