I recommend the following site for sizing your secondary:
http://www.bbastrodesigns.com/diagonal.htm
The last parameter is critical and depends on your tube radius + focuser height. Note aim for 70% illumination to match the field stop diameter (radius) of your lowest power eyepiece that you intend on using frequently.
eg. (mm)
250
1270
54
210
A 54mm secondary with 210mm to focal plane gives approx. 70% at 21mm off axis radius. A 31mm Nagler has a field stop of 42mm and is a perfect match as your lowest power eyepiece giving an exit pupil of approx 6mm. Your 100% illumination circle is 12mm. You would great greater illumination if you had a lower profile focuser or larger diagonal provided there was no vignetting anywhere in the path by tube or focuser. See also NEWT at
http://www.dalekeller.net/ATM/newton...t/newtsoft.htm