Obviously, holding too long is always bad.
Try reading (or re-reading) this article:
http://www.gun-tests.com/performance/aug96secrets.html and especially note the last section by
professor Lawton. Then go read this article that develops the drill further:
http://www.zediker.com/articles/zcomlawt.pdfI'd suggest that while Mr. Tubb's book is an invaluable aid and good reading as well, his approach to taking the shot can be misunderstood by novice shooters (especially if you're using a scope and shooting at big silhouette targets instead of 10 rings). I think you will improve faster by working on your
hold more than on your
approach.
At some stage you may be able to hold well (slow and small) and develop a control that will let you become pretty stable near the center then very slowly move to the center and break the shot at the same moment. But, for now, I'd suggest that you concentrate on refining your hold.
Develop a stable position, good natural point of aim, etc... Then as you hold, use Lanny Bassham's mantra from
With Winning in Mind, "slower, smaller". As you're waiting for the trigger to break, make this the
only thought in your conscious mind.
Also, before you shoot, change your objective from "breaking the trigger at the right time", to "getting into my small, stable hold". If the first is your objective, when you fail, it gets recorded on the score card (and reinforced in your mind!). When the second one doesn't happen, you get to start over and do it again. When it does, you'll find plenty of time to break the shot.
Next, go buy yourself an old target air rifle, enough said... Well not quite, get
Ways of the Rifle too.
Sam