The above syringe has medication (Ivermectin) for 1250 lb horse in 250 lb increments. To get even close to the 70mcg (microgram) dose of Ivermectin for your 20 lb dog you would need to take a dose for 1 ( 250 lb increment ) and divide it into 250 parts and then give you 20 lb dog one part. This will be very difficult.
If the only choice I had for Heartworm prevention was this syringe of Ivermectin paste , I would dispense a small dab about the size of a small green pea on the tip of my finger and give it to my dog as a best guess. Watch dog carefully to make sure there is no sign of toxic reaction. Please note that after opening this tube the shelf life will be uncertain and quality of dose questionable at best..........................
I have one question.....................Are you in the U.S. ?
I hope this helps , as the above is my best guess given your only choice.