What is the tap on a center tap transformer
A center-tap transformer is designed to provide two separate secondary voltages, VA and VB with a common connection. This type of transformer configuration produces a two-phase, 3-wire supply.

The secondary voltages are the same and proportional to the supply voltage, VP, therefore power in each winding is the same. The voltages produced across each of the secondary winding is determined by the turns ratio as shown.



See here is good video to how center tapped include and equally winding on both output side.

Showing how to calculate turns of winding, 
 How make the design of homemade center tapped transformer