Unless you do have a scalable ship icon and the GPS antenna location is accurately shown on that ship icon then you are guessing as to the positional accuracy on your chart display. Then you have the accuracy of the chart at the native scale to which it was drawn. The officially recommended method of sticking down Treadmaster is to use epoxy. I've found that less than satisfactory, with the glue breaking down at the edges and allowing the Treadmaster to curl. I used the following method which has held the material down perfectly on fresh gel coat with no sign of lifting after six years under the Australian sun.
Hard dinghy has less stablity, but does row easier.
I understand the boat of origin of the fire did have halon on board, but the fire may not have started in the engine compartment. I am certain that the boat next to it did have halon, but again the fire spread from superstructure to superstructure. The fact that you are new to boat buying makes you very vulnerable. You have no idea how bad these things can really get. So your defense is your surveyor and your diesel mechanic.