A short hop: via the E40, the E314 or the back roads
From Leuven your driver usually takes the E40 towards Brussels and leaves it at the Ring (R0), where the airport exit follows almost immediately. If you live in Aarschot, Rotselaar or Diest, he first follows the E314 until it joins the E40 near Leuven. From Haacht the N21 through Kampenhout and Steenokkerzeel is often the most direct route, from Tervuren the ride goes via the Brussels Ring, and in Tienen and Landen the E40 is practically on your doorstep.
Short does not always mean quick. On weekdays the E40 towards Brussels clogs up early, and a single accident near the Ring interchange can disrupt the whole morning. Your fixed price is not affected, but your pickup time should allow some slack. The journey time in the price table is a guide without traffic.