Turntable Leveling, Again

I may have asked this here before, but...I'm in the proces of moving my music room and am having a HUGE amount of trouble getting my Technics SL-1800 level. I have it on a homemade wall shelf constructed of pine and metal braces. The shelf is as level as I can get it. The strange thing is, using a cheap see-it-both-planes level from Lowe's, at the center of the platter I'm way out, but if I move the level to the edges I'm in. This would mean to me that my platter's not flat...is that the case with the SL-1800, or do I have a big problem? I have another turntable sitting beside it I use for 78s, and it leveled up in about a minute, so I'm not sure what part of this is throwing me off. I have a circular level as well; would I be happier (and less frustrated) with a metal torpedo level, do you think?

Much appreciated,

John Sellards