Update: Through a painstaking update to the Espoo internals, this problem has been essentially eliminated. Ignore the comments below.
When you’re holding button 6 (or RB), Pokey should release automatically when the parabola passes through the cross hair. The problem is the at it is not perfectly precise. This is due to details on how a digital computer simulates physical systems.
However it possible to account for the imprecision. You may notice that the auto release always releases the projectile just a little bit late. If you account for this lateness in your aim, then you might be more successful.