I use this to calc resistors in parallel: http://www.1728.org/resistrs.htm
What I do is measure the tailcap amps, say for example you get 3A with the two stock R160's.
Using Ohm's Law (V=I*R), solve for V: 0.08 * 3 = 0.24v
So, add one more 0.160 ohm resistor, you get: 0.0533 ohms, solve for I:
V = I * R, 0.24 = I * 0.0533, I = 0.24/0.0533 = 4.5A
So if you desire to get 4.5A, you would need to add another R160 or equivalent (50% bump). I say equivalent, because you could add two R320's which is the same as one R160. To go more mildy, you would add a higher value resistor instead:
So, adding a 0.200 ohm resistor (R200), you get: 0.057143 ohms, solve for I:
V = I * R, 0.24 = I * 0.057143, I = 0.24/ 0.057143 = 4.2A
So, adding a R200 resistor will result in a 40% increase in amps. Pretty easy? Don't you just love when the math works? I've done this stuff before and it comes out exactly with the bump I expected....
Problem is, of course, having the Rnnn resistors (current sense) laying around. I ordered a few different sub 1 ohm resistors a while back in the 1206 size from DigiKey. This is one example: http://www.digikey.com/product-detail/en/RL1632R-R160-F/RL16R.16FCT-ND/714519