I like to keep the coolant level down about 3" from the top of the radiator, this gives the coolant an area to expand.
I do not run the radiator cap locked down, on my to35, with a good clean radiator and clean coolant the tractor never gets hot and there is less pressure on the radiator. I think you only run about a 7 psi cap on the radiator, you need to check and see what psi cap is on your radiator.
If someone has placed a high psi cap on your radiator, this will build up too much pressure on the system and believe me your old radiator will not like high pressure. it will blow the coolant out.