The ForeSight spaceship rules had a drive mass and endurance table, implicitly including both motor and tank. When I came to build a spreadsheet to do all the fussy calculations (ForeSight had a formula for the mass of the structural frame) I decided to replace the table with Tsiolkovsky’s equation and treat tankage explicitly. It emerged that a lot of the drives implied superluminal specific impulses. So (for my own use) I replaced that chunk of rules entirely. I basically treated everything as a torch (i.e. fuel and fuel alone used as propellant), with TL6 drives based on the theoretical limit for fission fuels, TL7 on fusion fuels, TL8 on 1% antimatter, TL8.5 on 3% antimatter, TL9 on 12% antimatter, TL 9.5 on 25% antimatter, and TL10on 50% antimatter.
And that was all perfectly okay until that stinker Winchell Chung created his beautifully-researched Atomic Rockets website and let just anyone see what plausible speculation suggests for exhaust velocities and thrust-to-mass ratios.