I have a Futaba 9C and have the same "problem". I solved it by using a P.MIX curve. In the advanced menu, the last P.MIX has a curve. If you set channel 3 (throttle) as both master and slave (thus mixing it to itself) you can use the P.MIX curve to adjust throttle servo movement. If the curve is below zero, you will have less than "normal" (as set by end points) at that point, and above zero will give you more power.

To move the "hovering area" from 1/3 to 1/2, I would program negative values on stick position 2 and 3. Leave position 1 and 5 at zero (to keep the end points where they are), and let 4 be somewhere between 3 and 5 to smooth out the curve.

Exponential will not work in this case because the half throttle point will remain unchanged. Expo only works from half stick and towards each end. Since we're talking about moving 1/3 stick to 1/2, I prefer to use a P.MIX curve instead of expo. OTOH expo works well if you want to change the curve near the end points.

If you want to set the throttle curve "visually", you could also add another P.MIX that mixes 100% negative throttle to itself. This will cancel the ordinary throttle servo <-> stick link, and the P.MIX curve alone will control the throttle servo. To use this setup, the curve must be different, going from idle (0%) in stick position 1 to full throttle (100%) in stick position 5. Again points 2 and 3 must be a little lower to move the hovering rpm up to 1/2 stick.

Feel free to ask again if I have forgotten any details or my explanation is impossible to understand. I hope this helps more than it confuses.

edit: typo