DropShip Engines

In FASA's 1980s DropShips and JumpShips sourcebook, the engines on the Triumph, Condor, Fortress, Overlord and Excalibur all equal [ship mass]*[thrust]/15 - 50, and the engines on the Seeker, Monarch and Behemoth are close to that.

The Avenger, Buccaneer and Union are at [mass]*[thrust]/15 - 75, while the smallest ships, the Leopard, Fury and Gazelle are right around [mass]*[thrust]/15 - 100. The curve

[mass*thrust]/15 - [(14494*mass*thrust - 11150000) / (311*mass*thrust - 1279000)]

fits most of those engines. I don't expect to fit them all perfectly--when I worked through the JumpShip construction data I found typos, copy errors, and weirder inconsistencies which make that impossible. I'm pretty confident that the Mule and Mammoth engines have a digit wrong, and that the Vengeance's engine was calculated from a thrust of 3, instead of the thrust 4 its listed at. (I'm not the first to notice this about the Vengeance, but danged if I can remember who I've heard it from before.)

I'd feel pretty good about my equation, except that the ships which don't fit are all fast- the Achilles at thrust 8, the Seeker at thrust 5, and the Intruder at thrust 4 (in a band dominated by thrust 3 ships). So I don't think I'm accounting for thrust correctly.

I'd hoped to solve this by Thanksgiving, but que sera sera.