My indy rad shop said his rule is 10 degrees F top to bottom for one in good shape. One of the common practices of previous owners was to become frazzled with overheating Disco, pour in a double bottle of Wonder Wartz Leakz Stopz, which sealed up the tiny pin hole leak. And the rest of the container sunk by gravity to the calcium covered lower tubes, and with some sludge there formed more solid material, that keeps the water from flowing. Row after row, up we go, each time reducing the flow another step. It gets real fun when you pass the mid point. Hot air thru radiator has to warm up the face of the fan clutch, so it will shift from low power to full power. If a radiator is clogged enough, it keeps the fan from warming up, the fan stays on low power, and you overheat extra fast.

Being that we are warming up in the Northern hemisphere, we should be finding a lot of "marginal" radiators any day now. Just like in the first cold days of Fall we find lots of wimpy batteries.