No need for any calculation the Resistor with each 3 LED strip has already been calculated, however it will have been calculated to give a bright light from the LEDs which may be too bright for your purposes. I am afraid dimming the LED strips is a matter of trial and error, LEDs do not dim linearly so adding a certain resistance does not say dim the LEDs by half. If you can lay your hands on a 5K or 10K linear potentiometer put that in the positive lead as Brian has indicated and twiddle with it until you get the brightness you need. Take it out of circuit and measure the resistance between the two terminals you had connected (which should be the centre terminal - wiper and one either side) Then obtain fixed value resistors of the same value as the reading. Do not worry about blowing the LEDs as the onboard resistor limits the current to prevent that. As Brian has already indicated treat each 3 LED strip as an individual light source even if you intend to put more than one strip in a building, if you start using them in 6, 9 &12 or more than the additional resistor value will change each time. Finally if you have a multimeter measure the current drawn by an individual strip with your added resistor, by adding up the total currents drawn by all you lighting you can ensure your PSU can deliver sufficient current. It is good practice not to run PSUs constantly at their maximum output and I would advise you to leave an overhead in the region of 10-20% spare.
Richard
) but in the absence of a specific answer surely a bit of trial and error is called for.
