They are mongrels to get right, they don't follow any particular line of the tank to use as a reference,this is how I done it.
First get some good original pics to use as a reference,have the tank on the bike (near impossible to get right off the bike!) then make sure the bike is sitting level, I had my bike on a workstand then put blocks of wood under the wheels to level it up (or get an assistant to hold the bike), I used the Honda logo on the clutch cover as reference to the bike being level,Get some of the blue low tack masking tape & stick to the top & bottom edge of the decal, then position the decal where you think it should be,you can then stand back & check if its right or needs more tweaking,when you are happy run a strip of the tape on the tank underneath the decal for height reference & mark with a grey lead where the decal finishes or starts for sideways reference on the tape,remove the decal, peel off part of the bottom & cut a small part of the backing with scissors,then position the decal where you have marked,stand back & check again that its sweet before peeling off the rest of the backing & sticking down
