50%, like all other bonuses, is added to the base strength.

So, if you have Marauders boosted (with research, prestige, and other stuff) to 80, the 50% still won't be added to that, but to their basic off, which is 40. That means, 50% = 0.5 * 40 = 20 extra, and you'll end up at 80 + 20 = 100 in total, which is only 25% bonus of the boosted strength (80). So, it may look wrong to you, but that's because you're looking at the wrong base for applying the 50% bonus.