Eric, there is another vignetting issue... you may be OK at the middle of your 350D sensor, but not at the edges...
another argument towards larger secondary (while having low profile focuser as well).
Newt software (
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm) proposed by Geoff may help a lot here. It certainly helped me :-)