No Problem....the kettle doesn't have gallon marks inside, but I found this link awhile back for measuring:
http://merrycuss.com/calc/kettle_volume.html
That'll work. There will be many more beers brewed in the kettle. I just did a breakfast stout the other week and now think I'll do a ESB for the wife this weekend. Either way Spike kettles are awesome and well worth the wait and price.
I use a Rubbermaid cooler 10gallon. What I do is preheat with 100 degree water drain it, bring the water up to about 170 dump it back in. Then I dump my grain bill making sure there aren't any cool pockets. The stationary thermometer which you have is nice, but it only checks the temp in one...