Congratulations on your Tele Vue! The 24mm Panoptic and 13mm Nagler is a very nice combo and the 2.5x Powermate does give you access to higher powers with both. Your next choice will be more difficult though as you do not really need anything between the 13mm and the 24mm but if your were to say choose the 16T5 or 19Pan (same true field) you will get quite different powers when combined with the 2.5x Powermate if the high power combo is what you are after.
Now that you have tasted Tele Vue, save up for a low power 2" eyepiece even if it is a 26mm or 31mm Nagler (and please keep the 24Pan as it is a most versatile eyepiece in any telescope). If you were impressed with the 24 Pan and UHC, just wait until you see the Eta Carina nebula with an O-III filter and 31mm Nagler!
If you want even sharper stars at the edge you can add a Paracorr unless you don't find the coma with 12" at f5 objectionable.
For true field calculations see
http://www.televue.com/engine/TV3_page.asp?id=89
or create a spreadsheet for yourself and add eyepieces and telescopes as you aquire them. You can get the Tele Vue eyepiece parameters at
http://www.televue.com/engine/TV3_page.asp?id=28
You need the field stop diameter for calculating true field. If the eyepiece has the same field stop diameter it will give you the same true field in a given telescope.
You have made a wise choice, enjoy!