dump was written to assume 9-track tapes, so some fudging has to be done
for DDS tapes.  The following has the info you need along with several
alternatives for dump parameters.

Approximate capacity of 60m DDS tape = 1.3G bytes
Approximate DDS tape density = (1.3G bytes) / (60 m) = (550K bytes/in)

dump assumes an inter-record gap (IRG) of 0.3 in for density = 6250,
0.7 in otherwise.

dump uses a default blocking factor of 10 for density < 6250,
32 otherwise.
================
density = 550000
blocking factor = 32 (default)
assumed IRG = 0.7 in

Block length = (32K bytes/block) / (550K bytes/in) + (0.7 in) = (0.76 in)

Effective tape length =
        (1.3G bytes) / (32K bytes/block) * (0.76 in/block) = (2511 ft)
================
density = 6250
blocking factor = 32 (default)
assumed IRG = 0.3 in

Block length = (32K bytes/block) / (6250 bytes/in) + (0.3 in) = (5.54 in)

Effective tape length =
        (1.3G bytes) / (32K bytes/block) * (5.54 in/block) = (18325 ft)
===============
density = 1600
blocking factor = 10 (default)
assumed IRG = 0.7 in

Block length = (10K bytes/block) / (1600 bytes/in) + (0.7 in) = (7.10 in)

Effective tape length =
        (1.3G bytes) / (10K bytes/block) * (7.10 in/block) = (75113 ft)
===============
density = 1600
blocking factor = 32
assumed IRG = 0.7 in

Block length = (32K bytes/block) / (1600 bytes/in) + (0.7 in) = (21.18 in)

Effective tape length =
        (1.3G bytes) / (32K bytes/block) * (21.18 in/block) = (70022 ft)