My scenario is to create a dc power supply circuit(Vout 3-15vdc) capable of 40A peak load. MUST use the below described TOROIDAL power xfrmer:

Input Voltage: Series: 230VAC, 50/60Hz
Parallel: 115VAC, 50/60Hz
Output Voltage: Series: 100VAC CT@ 10.0A
Parallel: 50VAC@ 20.0A

I need to stepdown to 30vac or less to allow for options in regards to regulation, protection, linearity, etc...

Can I use only 1 side of primary @115 and put both secondarys in series to get 25vac?

Are you trying to answer a theorectical question or an assignment question?

I don't want to give away the answer... but your last question describes
The solution, almost...

Try drawing the diagram and labeling the primary and secondary,
With some basic math the answer should fall out? You are close...

(above comments intended for theoretic discussion "only")