Here is the rough lines for 50% visibility(by surface area)
First: the top, Blue line, is for any dreads with a gun-arm, it is slightly higher for 2 gun-arms; this is because weapon barrels do not count, but
DCCWs should(they are not weapon barrels). For the 2-gun Dreads, the line should go across the bevel-line just above the Blue line.
The second, Purple line, would be for 2x
DCCW dreads; since the Arms are not "Gun barrels", they would be hull and therefor are portion of overall visibility.
These are not exact measurements, just somewhat rough "eyeballs" to give a reference in game, exact situations will vary and you and your opponent will need to discuss/look, in-game, whether a Dread is obscured or not(If you cannot reach a consensus quickly, default to the Cover-1 rules for Contested Cover).