Thrust in the Cruise
Easiest way to do a quick estimate is to use the Lift/Drag ratio. In steady level flight (i.e. constant speed, height and direction) Lift = Weight and Thrust = Drag.
Typically, the weight at the start of cruise = 0.95 x takeoff weight. This accounts for the fuel burnt during takeoff and climb.
In the cruise, apply L/D = W/T and rearrange:
T = W / (L/D)
L/D in the cruise for most jet transports is in the region of 15, so in simple terms thrust required will be Take of Weight x 0.95 / 15
Look at the A320-200 as an example: MTOM = 78000kg = 764.92kN
Maximum sea-level static thrust = 240kN total for the two engines
764.92 * 0.95 /15 = 48.44 kN, which is about 20% of the rated sea-level static thrust for the 2 engines together. I suspect for most modern commercial jets the % rated thrust figure will be similar.