My 620 ran just below 1/2 in the marked 'run range'. Sounds normal.
For good cooling you need good air flow and good coolant flow.
Be sure you have a fan shroud. It increases the fan efficiency
Make sure the rad fins are straight and clear of bugs and debris
Make sure there are few areas around the rad for air to get past without going through it. One is under the hood just above the rad support. If there are gaps seal them up.
Clutch fan should turn by hand but stiffly. If very loose it will not move enough air.
If you don't have a coolant recovery system get a later rad cap and a pop bottle. Coolant expelled when hot is recovered and stored. When the engine cools down the coolant is sucked back in. Rad rarely needs topping up.
Always run 50/50 distilled water and anti-freeze mix.
Check for soft rad hoses. They rot from the inside out and may look ok and pump suction will collapse them.
Make sure the heater hoses are not joined together to bypass a leaking heater core.
Cap off and coolant level down enough to expose the top of the core... are the tube openings crusty with deposits from hard water? This will reduce flow.