I would suggest trying a logarithmic scale. This is probably best as a power factor. As they drag further away, the power will increase at a slower rate.

For example:

var maxPower = 300;
var scale = 4;
var distance = (the number of pixels they dragged...);

var power = maxPower * log( distance / scale );

scale image http://upload.wikimedia.org/wikipedia/commons/b/b9/Comparison_of_the_sequence_1_to_10_and_their_logs_to_the_base_10.png

I would suggest trying a logarithmic scale. This is probably best as a power factor. As they drag further away, the power will increase at a slower rate.

For example:

var maxPower = 300;
var scale = 4;
var distance = (the number of pixels they dragged...);

var power = maxPower * log( distance / scale );