You can. That's why a refractometer is handy. Generally in AG brewing we take a pre and post boil reading, because we usually know our boiloff rate. But you can take readings and boil til you get to your intended gravity.
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Revvy's one of the cool reverends. He has a Harley and a t-shirt that says on the back "If you can read this, the bitch was Raptured. - Madman
I gotta tell ya, just between us girls, that Revvy is HOT. Very tall, gorgeous grey hair and a terrific smile. He's very good looking in person, with a charismatic personality... he drives like a ****ing maniac! - YooperBrew