What I'm suggesting is keep doing G29's with a bigger and bigger spread of F & B until you have the full Y axis being probed. (Or... As much of as you can because you have fairly large offsets.) Then do G29's with a bigger and bigger spread of L & R values. At some point, you will know what the correct values are. When you know this information, you can edit it into those #define's in your Configuration.h file, compile and load it. At that point, the bare G29 command will have the information it needs to probe a large portion of the bed without specifying L, R, F, and B parameters.

You can issue the G29's by using PronterFace.