If your drawings are not accurate, as many are not, then the only way to locate the D-Box is to probe. The D-Box is going to be at about the same level as the outlet pipe on the septic tank. This gives you an idea of the depth you are looking. The best way is to get a metal rod. It should be stiff so that it does not bend in the ground. (If you have a plumber friend, he might have one.) It is easier it either has a "T" type of handle or if you can add one. You can purchase something like this at some hardware stores or a plumbing supply store. If you cannot find one, you might get a piece of metal metal rod, perhaps about the size of your little finger and a piece of conduit connector which is a "T". You can then put a handle through the connector and attach it to the rod. The next step is to find the outlet pipe from the tank. Gently probe along the pipe and follow it until you find a "box". Generally this is about 12-15 inches in square. You generally find this within 6-10 feet of the septic tank and in a straight line with the inlet and outlet on the tank. However this is not always the case. I just had one yesterday where the box was offset by about three feet. The best advice is to be systematic in the probing, and be gentle. You do not want to break any pipe. If you have any questions, please do not hesitate to call.