I’m not quite sure what your asking but I’ll attempt to answer. Firstly the eyepiece field stop is at the focal plane of the eyepiece which when placed at the focal plane of the telescope will produce an image in focus and magnified by the ratio of focal lengths. A 31mm Nagler with an 82 deg apparent field has a 42mm field stop diameter. Your 30mm eyepiece with 43mm field stop diameter will have a slightly larger than 82 deg apparent field but it is also dependent on % distortion present in the eyepiece.
You also mention illumination of the field (also referred to as magnitude drop) and the goal here is to size your secondary mirror to give you about a 70% (Newt uses 75%). Mel Bartels used to quote 70% but now uses magnitude drop (0.4 as being acceptable).
I use the Mel Bartels diagonal sizing program and aim for 0.4 mag drop for my widest field stop eyepiece I use the most. A slightly greater than 0.4 mag is hardly perceived especially for a wide cone of light from a fast mirror. If you are a variable star observer you want the widest true field you can get (46mm FSD) and zero magnitude drop over that field. If you are a dedicated planetary observer you can drop a bit more provided your 100% diameter is not too small. (Old rule of thumb was aim for 12.5mm and this still works but typically these days with very wide field eyepieces it’s nice to have a larger fully illuminated field).
See
https://www.bbastrodesigns.com/diagonal.htm
I personally prefer % illumination instead of magnitude drop and aim for a 70% drop at the edge of my widest FSD eyepiece but I have a couple of telescopes giving me 60% with my 41mm Panoptic and I can hardly tell the difference at the edge of field.
Note distance from secondary to focal plane can be adjusted at the design stage and sometimes after and this will affect % illumination.