If you look at page 14 of the LimeSDR-USB schematic, the reference clock in has an impedance of ~50 ohms.

So basic electronics 101: V=I * R

If you are driving with 8mA that would be 0.008 A * 50 ohms which is 0.4 volts

If you are driving with 16mA that would be 0.016 A * 50 ohms which is 0.8 volts

If you are driving with 32mA that would be 0.032 A * 50 ohms which is 1.6 volts

Iād normally start with the lowest driving current and dial it up until it works (stopping at 66mA). I do not think that there is any need for an attenuator, not unless you plan on feeding the reference clock into one of the the RF ports instead of the U.fl J19 reference clock in port. The datasheet for the ADF4002 gives the minimum REFIN Input Sensitivity as 0.8 volts peak to peak, so maybe start at 16mA.