To use the calculator:
First, select the core material type.
Select Iron Powder for cores with a 'T' prefix (such as the T-50-2).
Select Ferrite for cores with an 'FT' prefix (such as the FT-43-63).
For more information about using this calculator, see Using the Toroid Winding Calculator.
Two great references on toroidal inductors and transformers:
The ARRL Handbook for Radio Communications
Transmission Line Transformers