The bias winding is also being used as a shield - the bifilar winding with larger wire will help to maximize coverage to make a better shield.

As I said, this approach is not right, at least a text should appear to explain why this Bias is using 2 wires in parallel even is not usefull at all from the electrical point of view. Also, some designs is calculated for nonstandard gap ferrites. I am sure nobody is willing to play adjusting the gaps to a specified value. Solutions given by PI is just  theoretical, not physically viable. Ferrites should be chosen from existing part numbers not resulting from some algorithms. As size/format. and PI part numbers

Using the bias winding as an additional shield is standard procedure in power supply design. Also, non-standard gaps are used routinely by high-volume supply manufacturers so that they can optimize their design. They have the ferrites custom ground by the core manufacturer or an outside contractor, and enjoy economy of scale. A standard ferrite gap/AL might not yield an optimized design. We at PI have in house a core grinder  that is used to custom gap ferrites to optimize our in-house designs.

If you wish to use a commercial core with standard gap/AL values, it is possible to manipulate the KI and KP values in the design spreadsheet to come up with a design that has a gap/AL close to a standard value. It may also be necessary to manipulate the number of secondary turns in the design.