How to measure (using .MEAS) with LTspice?

I find behavioural voltage sources invaluable for this sort of stuff - using maths functions in LTSpice to do the work for you. Select a new component as normal, but instead of picking a 'voltage' pick 'bv'. This component will produce a voltage according to some maths function. Right click over the bv component and in the Value box, type in V=V(out)/V(in) (I have relabelled your input as IN and your output as OUT). Plotting the output of the BV will give a real time value of what you are looking for. Attached is your circuit. Here is it mentioned in one of my tutorials (good for calculating instantaneous efficiency):
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice_tutorial_3.htm

Good luck - Simon