Thanks for that info Jason
. I should have mentioned that it is also capable of 2" eyepieces if that makes a difference to what you have suggested. I will do some research into that Panoptic and the Nagler 13mm. Not sure if I have done the calcuations correct or not, but it shows the Nagler as having a smaller true field of view than the current 25mm plossl. Also, is the 12mm eye relief going to be an issue? Though she does not wear glasses.
I have attached an image to my spreadsheet showing my maths on the various eyepieces.
Thanks again for your input and help.
http://www.rangiorafire.org.nz/scopesheet.GIF