Putting it all together

Inputs

angle, angle_rate: the tilt angle of the scooter in radians and its derivative in radians/sec 
steer_knob: the reading from the steering knob, between -1 and +1.

Balance

balance_torque = 5.0 * (angle - rest_angle) + 0.4 * angle_rate

Limit top speed by tilting back

overspeed = max(0, cur_speed - 0.5) 
if (overspeed > 0) { 
  overspeed_integral = min(0.4, overspeed_integral + min(0.2, overspeed+0.05) * dt) 
} 
else { 
  overspeed_integral = max(0, overspeed_integral - 0.04*dt) 
} 
rest_angle = 0.4*overspeed + 0.7*overspeed_integral

Steer. Decrease steering rate at high speed

steer_cmd = 0.07/(0.3+abs(cur_speed)) * steer_knob

Track current speed

cur_speed += 1.2 * balance_torque * dt

Differential steering

left_motor_pwm = balance_torque + cur_speed + steer_cmd 
right_motor_pwm = balance_torque + cur_speed - steer_cmd

Outputs

left_motor_pwm and right_motor_pwm directly set the duty cycle of the pulse width modulator for the wheel controller, and range from -1 to +1 (+1 is 100% forward, -1 is 100% reverse.)