Yep I have 40 + years of real world experience. I was an AC Power Distribution Design Engineer and have a BSEE as part of my "formal education" as you call it from good old Purdue University a top notch Engineering School, so not to brag but YEP I DO KNOW SOMETHING ABOUT THAT LOL
To answer your question:
In a 120/240 Volt Three Phase Four Wire center Tapped DELTA, IF L2 is the Red/High Leg then:
Its 240 Volts L1 to L2
Its 240 Volts L2 to L3
Its 240 Volts L3 to L1
Its 120 Volts L1 to Neutral
Its 120 volts L3 to Neutral
Its 208 Volts L2 to Neutral (Red/High Leg)
Its the transformer between L1 & L3 thats center tapped with its center grounded and that point is the Neutral
In a 208 Y 120 Volt Three Phase Four Wire System
Its 120 L1 to Neutral
Its 120 L2 to Neutral
Its 120 L3 to Neutral
Its 208 L1 to L2
Its 208 L2 to L3
Its 208 L3 to L1
So thats about as plain as I can describe it to a lay person sure hope this helps. If some "Professor" says different then Hes wrong and Im right is all I can tell you. NOTE sure there can be some slight different voltages (204, 206, 208, 210, 212 etc etc) depending on resistances and loads and the tap settings etc. etc. Im unsure that "Professor" understands the difference in a Delta versus Y configuration???
NOTE do NOTTTTTTTT feel bad if you cant understand this and sorry I can NOT put in a paragraph here what takes volumes of books to explain and years of engineering practice and experience to fully comprehend. I try my best to help and educate the fine gents here but I cant make them understand volumes of theory and years of expreience here in a few sentences BUT BY GOLLY I DO TRY MY BEST.
God bless you for your good thoughtful questions and sparky chat and I sincerely hope I helped you understand all this, sorry if I couldnt.......
John T BSEE, JD