Manometer measures the ACTUAL draft of the stove, you cannot just "guess" based on flame height or anything for that matter. It is not a one time deal, your draft should be checked every heating season, conditions, blowers, piping, ash buildup all have slight effects on it.
ALSO, you need it to set up your Baro damper correctly so it operates and keeps the correct draft on the stove.
Do a Search on Manoometer, this is a very discussed topic.
There is a Loaner Program for the Manometer, but you can also get them online for $20-25.