As you've no doubt discovered you can set the position of an un-modified servo by using the act_setSpeed command. This specifies a location which can be:
DRIVE_SPEED_MIN for say fully anti-clockwise, (this value is -ve) and
DRIVE_SPEED_MAX for fully clockwise (this valie is +ve).
The mid-point, also called DRIVE_SPEED_CENTER, represents the neutral centered position and will always have a value of 0.
These names are just shorthands for numbers. The key being that a value of '0' represents center.
So to move a servo halfway between 'center' and 'fully clockwise' then you set a speed of:
('fully clockwise' - 'center') / 2
(DRIVE_SPEED_MAX - DRIVE_SPEED_CENTER) / 2
but since DRIVE_SPEED_CENTER is always 0 then this can be simplified to
DRIVE_SPEED_MAX / 2
Another way to work is to work in percentages - if you are more comfortable with that. ie -100%=Full anti-clockwise and +100%=fully clockwise.
Your program then generates a number in the range -100 to +100.
You can then use the 'interpolate' function (check the docs) to map the value from -100 to +100 into the range DRIVE_SPEED_MIN to DRIVE_SPEED_MAX as follows:
int8_t myPercent; // Assume this variable has your setting from -100 to +100
// Convert myPercent from -100 to +100 into the DRIVE_SPEED_MIN/MAX range
DRIVE_SPEED speed = interpolate(myPercent, -100, 100, DRIVE_SPEED_MIN, DRIVE_SPEED_MAX);
// Now set the servo location/speed
Hope that helps