My first attempt at sending this went to you personally and I really wanted it go go on the Forum. So at the risk of repeating myself………
There are a couple of things you could try. Option 1 – I guess lifting the boat out to check the off-set it not an option. Option 2 – When you are next in shallow water, preferrably moored up, where you have a positive reading on the sounder, drop a line over the side to check the depth of water. The depth of water, less your draught is the depth that should appear on the sounder. You should be able to calibrate based on any discrepancy.