You are asking for more than double what they gave you to start and you have only had your card for three months. Give it some time and perhaps ask for a lower amount next time.
+1 to the posters above. NFCU is an amazing CU to have a relationship with. I started on the lower end with them when my credit scores were lower 600's. In about three years time I have managed to improve my relationship with them to 20K+ credit card, 15K NAVCHECK and just refinanced my mortgage to them last month. The credit card was all auto CLI's. If you do well with them you will probably see one around the 6 month mark and again at a year. They seem to have steps they take you up in but I would not be surprised to see you at 5 figures CL after a year.
I think the fact that a CLI was achieved at the three month mark says much. Just give them some time to get used to your spending patterns and see that you can use what they give you responsibly. Next thing you'll know, you'll be posting about exorbitent increases like everyone else and suddenly the coveted 5 digit limits won't be so impressive anymore. The LUV button will just be a Saturday/Sunday/Any day thing to press and then they'll catch on and we'll all be shunned from getting increases like normal folks who wait do. NFCU will take care of you if you take care of them. Just give it some time. All jokes really...I truly believe that with NFCU being as big a lender as they are, the increases they've been handing out are ones they are comfortable with giving per person.
I really think that magic number is already predefined based on their reviews of your account.