One other thing you may run into is an error when trying to set a custom WB with your A700 (usually caused by "hot spots" from too much light reflecting from what you're measuring it from).
That's one reason something like white coffee filters can work in a pinch (they're not as likely to have that issue as something else white if you don't have a gray card handy).
Another trick that works with the A700 is going manual focus and deliberately throwing the lens way out of focus before setting it. The camera doesn't care if it's got focus lock or not, and that helps to diffuse the light when taking a reading.
You'd probably laugh at how I sometimes set my WB. I've got a camera bag that's gray inside, and I've found the inside of the top lid works pretty well for that purpose, giving me slightly warmer colors than you'd get with a more accurate WB setting (which can sometimes be a bit on the "cool side" compared to what might look at bit nicer). LOL