Access to wind tunnels and aerodynamicists are the preserve of the pros, but with a few simple tools and some testing you can get a decent approximation of how well your set-up is working.
First, try reducing your frontal area. The simplest way to do that is with a mirror and a pen (lipstick’ll work if you can’t find a non-permanent felt-tip). Set your bike up facing a mirror, climb onto it and tuck yourself into your aero position. Then, with the help of a willing volunteer, draw an outline around your reflection in the mirror.
Are there adjustments you can make that will allow you to adopt a new but comfortable position with a smaller outline? Can you drop your bars any further or reduce the width your extensions are set at? Even something as simple as holding your head lower will make a difference.
To test the effectiveness of any set-up you need a route that you can ride at as constant an effort as possible (a heart rate or power meter would come in useful here). The fewer hills there are the better. Time yourself over the route with your existing position. Then, on another day when you’re feeling as fresh or as fatigued as when you did the first test, ride the route again at the same effort but with your new position.
Provided the weather conditions and your effort level are as similar as possible on both occasions, any time difference is down to your new position. Then all you have to do is keep tweaking and testing until you find the position that saves you the most time.