Well, you've come to the right place!
Use a Field Controls Model RC baro damper.
Install a "tee" on the back of the stove with a cap on the open end so you can clean the ash out of pipe and behind the baffle in the fire box. That's where lots of fly ash will accumulate on the Mark stoves. Use another tee at the turn into the thimble, place the baro there. You will have to set it with a manometer, because it's at a turn in the pipe the settings on the weight scale won't be accurate.
Just remember that the baro can be installed in a turn in the pipe as long as the gases flow away from it. Never install it where the gases flow towards it.
Start the stove and build it up to a full vigorous fire, then set the baro damper. I run mine at -.05" WC.
Apologies to those who have seen this photo a thousand times, but it shows how my Harman is piped with the cleanout and the baro damper.
There should be no need for the manual damper on a Harman Mark with the baro damper. Field Controls also makes the RC in black, most suppliers don't have it in stock, they will have to order it for you.