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its to do with your draw tube length and what you need to make it clear the inside of the OTA. if your draw tube is say 100mm and you specify your imaging plane at say 90mm from ota it will drop 10mm into the light cone.
As for your fully illuminated circle yes you are correct. the main modifications that will increase your 100% is the secondary and focal point distance from the OTA. Don't worry about the 75% rays they don't matter if your illuminated circle is nigh on 100%
Just for your info my f4.7 uses a 70mm secondary and sits close to 300mm out of the tube for the focal plane i see as measured in CCDisp approximately a 6% light drop off to the outside.
Ide expect if you had about the same back focus then your secondary will be around the 80-85mm
Im not an expert in Newt, but these are the things i have come to agree on for myself and it hasn't let me down yet.
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