The approach I take for warships is to define a percentage of ship tonnage dedicated to engines for the fleet (may vary for fighters/FACs) and use the maximum engine boost at that tech level, since warships usually don't worry about fuel range that much the maximum boost is almost always optimal.
The techs for max EP modifier run out well before you're even halfway through the main engine techs, so the tech levels may not be exact in terms of what people consider TL1, 2, 3, etc. That being said, you can ballpark something like this using, for sake of example, 30% engine mass:
Tech Level 0: Conventional 1.0 EP/HS * conventional 1.0x modifier * 0.30 * 1000 km/s = 300 km/s
TL1: NTE 5.0 EP/HS * 1.25x modifier * 0.30 * 1000 km/s = 1875 km/s
TL2: INTE 6.4 EP/HS, 1.5x modifier --> 2880 km/s
TL3: NPE 8.0 EP/HS, 1.75x modifier --> 4200 km/s
TL4: INPE 10 EP/HS, 2x modifier --> 6000 km/s
TL5: Ion Drive 12.5 EP/HS, 2.5x modifier --> 9375 km/s
TL6: M-P Drive 16 EP/HS, 3x modifier --> 14400 km/s
TL7: IntCF Drive, 20 EP/HS, still 3x modifier --> 18000 km/s
After this the max modifier is capped at 3x, so the relative increase for the succeeding tech levels is not as sharp. You'll notice that the biggest percentage jump comes at TL5 and TL6 when the amount of the modifier jumps from 0.25 to 0.5 per tech level, which may be why you see people highlighting ion drives as a major tech breakpoint.
It seems to me that usually, the issues with speed on ship designs comes from designing to a specific tonnage and choosing the weapons loadout first, so settling on the engines first and fitting in whatever weapons you can on the remaining space ensures you have a good speed. Either too small of a space fraction for the engine, or forgetting to use the maximum modifier (not only do warships not need as much fuel/range compared to other types of ships, but extra fuel storage is usually less net tonnage than an inefficient engine) are probably the main reason you see a lot of designs that get criticized for low speed