From what I've seen, it is normally accepted for starting motors to pull down the voltage to 10.5 volts under load, way more than a 4% voltage drop. However, I'd suggest that using the heaviest possible cable, to provide the least drop and ensure the best starting power, is a better idea, if you can afford it.
From that point of view it may be enough to simply buy the heaviest cables
you can afford, that fit with correct sized lugs on them. And of course also making sure the lugs are properly crimped, which DIY cable lugs often aren't.
Or of course, there are those clamp ammeters, $50 will get you all sorts of "good enough" ones for this purpose. I'd still expect a 4% voltage drop to require heavier cable than you will want to install. Or pay for.