A 135 mm lense at F2.8 will give FL of 378mm
toucam has 5.6um pixels at 640 x 480 you get an image scale of 3.05 arc sec per pixel with FOV of 24.4 x 32.6 arc min
My guider setup is 210mm fl at f3.5. In order to callibrate this in PHD my calibration setting is 2500 assuming a guide rate of .5x sidereal
You dont need longer focal legth to guide your setup. see this article here:
http://www.wilmslowastro.com/tips/autoguiding.htm
with 5.6um pixels assuming you were aiming for 1 arc sec per pixel resolution anything greater that 190mm FL will do the job. (Thats assuming a mediam level of centroid accuracy. Ie between best and worst)
You may find you have to defous your guide camera very slightly if your guide star is only a few pixels wide as to calculate the centroid accurately the guide star shoudl be spread nicely across 4 - 6 pixels.