I’d second Hillbillyhort’s numbers. It depends on the exact type of lead-acid battery you’ve got (flooded, AGM, gelled) but 12.05v is around 50% charged or a bit less by my charts as well.And you will get different voltage readings right after charging or when a load is on the battery. So most accurate to take it a few hours after charging (either by wall charger or the tractor’s atl/gen) when its been idle for a while.
Regardless, your tractor batt is pretty far down whether at 50% or 45% or whatever. Be good to throw a charger on it for a while and get it back up to near full charge. It really makes starting in cold weather a whole bunch easier.