While It's tough to say anything definitive about the downhaul tension you're using now without some photos showing your rigged sail, I can say without question that a downhaul tool will make the rigging process a breeze and give you the subtle control that is difficult to achieve by hand, or even with a hand held tool.
There are a number of downhaul tools out there, and their ease of use depends greatly on the type of mastbase extension you're using. I have two different types. The one I have been using for quite a few years now is called a "Rig It Right", and it works based on a lever concept. It works very well with the older Fiberspar and NP mastbases. Arguably more well known and popular is the winch type offered by Chinook. Chinook's has two variant designs, one with the standard two pin cup and the other with the Euro Pin attachment.
Downahul tools cost about $40-45 US, and in my opinion, it would be money well spent. Believe me, your back will thank you for it. For years I manually downhauled with no apparent difficulties, but I ultimately began having back and related nerve problems. With the exclusive use of a downhaul tool, all my physical problems vanished.