You are right about the way to calculate distance by measuring the read to the contig ends.

From http://www.cbcb.umd.edu/research/contig_representation.shtml,

{TLE
src:1027
off:0
clr:618,0
gap:
250 612
.
}

The offset field marks the start of the read. in this case, off: 0 means the read is at the start of the contig. clr: 618,0 means it has 618 bases matching the contig, and in reverse orientation.

So from the offset you get the location of the read, then you can get the distance for paired reads. For scaffolding, you have to also pay attention to the read orientation with respect to the contigs.

contig1              contig2
=============        ============
    ---->               ---->

Then you need to flip contig2, since you expect the reads to point towards one another.

contig1              contig2(-)
=============        ============
    ---->                <----
    |------- distance -------|

It is not part of what you asked for. But if you are thinking of a scaffolding solution based on AMOS files, you are likely doing a similar thing as BAMBUS and BAMBUS2. BAMBUS looks at each paired read, then calculates contig links, and then bundles them into contig edges. It uses a bunch of heuristics to produce a linear order of the contigs. It is worthwhile to study the paper.