The calculator can help with the following questions:

1. How much tube is needed.
2. How big the coil should be.
3. With which diameters the space in the column is well used.
4. After how many windings or cm/inch the transitions to the next coil will be.

This ensures that you don't end up with too much or too little tube for the outermost coil. So you know before bending how the coil will look, and then when bending when the transitions will come.
By "cold finger" here is meant a simple return of the condenser tube from the innermost coil through its center.
"One coil without cold finger" can also be used to calculate product condenser coils.