It appears that the "-B records" flag actually takes tape size in kilobytes,
rather than "dump records" (at least the -b option has no effect on -B
except to tell dump how to round down the tape size).  Also, the claim that
-c modifies the calculations to be "more appropriate for cartridge tapes"
is a rather curious one; I'm not sure which cartridge tape is sets the
default density and tape length for (certainly not mine ;-), but it also
seems to add in extra wastage to account for streaming behavior -- however,
MY streaming cartridge drive doesn't waste any space (time, OH YES it wastes
time if you don't keep it fed with data), which I think is how all QIC drives
work (though I do remember a streaming 9-track drive whose IRGs could get
annoyingly long if you were just barely slow enough...).  At any rate, the
documentation should probably describe what -c really intends to do (assuming
that anyone actually *knows* ;-).