The UTM in this case probably went too far in the simplification since the math from 3rdR isn't anything that should cause people to break out in hives, it is just very poorly writen out in the 3rdR rules. Also 3rdR made it unnecessarily complex with its "treat as 1 hullsize smaller" a statement; that should be dropped. Just add up the HS of tug and whatever, compare to the chart to find equivelent warship hull, look at the number of Ic engines on your tugs and determine speed normally. That is a much more straight forward way to make it work. For over 500 HS limit speed to 1, and scale engines with HS so 1500 HS would take 45 Ic engines (3x15).
There is an aweful lot of rules in starfire that are for some reason or another significantly more complex then they need to be, I'm not sure why. But tractors and pressors are really the poster children for "get a proffessional rules writer from Avalon Hill to fix this" in my opinion.
There is a difference between 8 DD sized tugs (or more likely 2 BC sized ones) and what amounts to 2 DD sized tugs or a single cruiser sized one so I think the UTM simplification went too far. But I also doubt it comes into play often enough to make it a real issue.
You're welcome it was an interesting question it is not something I think about often (trying to move Theban style AB's).