Have a look at the specs at http://amsat-uk.org/funcube/funcube-cubesat/
Basically it is an inverting transponder which means that a signal received at the top end of the uplink will appear at the bottom end of the downlink. Also note that the referenced web page shows that the uplink is LSB while the down link is USB - which makes sense for an inverting transponder.
By the way, you should not be hearing a signal on 144.960 - I will use 145.960 instead and for now ignore the doppler shifts which you can take into account as needed for yourself (as you would with any satellite work).
You will note that the frequency range of the uplink is 20KHz - from 435.130MHz to 435.150MHz - as is the downlink - from 145.950MHz to 145.970MHz. Therefore, if you are hearing a signal on 145.960MHz, subtract 145.950 (the bottom end downlink frequency) from it and then subtract that figure from 435.150MHz (the top end uplink frequency); 145.960 - 145.950 = 0.010; 435.150 - 0.010 = 435.140MHz.
To choose a frequency that does NOT happen to be in the middle of the passband: if I hear a signal on 145.955 then... 145.955 - 145.950 = 0.005; 435.150 - 0.005 = 435.145MHz.
Of course you need to be aware of the deviation of the signal and so avoid the bottom end of the uplink so that you do not transmit out of "band" of the satellite.