Here's a trick that I read somewhere. If you stick to a standard type of control horn, you can
use one as a guide. That is, snip off the threads on two screws so that just a small amount
of thread is protruding from
a spare horn. Position and mark the first hole for the new control horn.
Then use your spare with the two shortened screws to mark the second hole.
Place the screw in the marked hole and press down which will position/mark the second hole.
Then drill accordingly