I am not bashing on you here, but I am somewhat surprised that you have completed your education and haven't heard of bracketing the GLA (and presumably, also, having comps with sales prices that eventually bracket your value finding). This was taught to me in the late 80s, so I would've assumed that they were still "preaching it" today.
I agree with Mr. Boyd though, it is one of those "scientific" aspects of appraising that some UWs sometimes hold too near and dear, when in fact it is not always possible. And when it isn't possible, it shouldn't be as apocalyptic to the process as they often make it sound. When I can't bracket the GLA, I have taken to devoting a lengthy comment to it in an effort to educate some of the newer UWs out there that think every home has a perfect set of bracketed comps available for it at all times. Sometimes it works, sometimes it doesn't. Sometimes I hate appraising...lol