ex902,
Welcome to PPRuNe!
Your Flight Planning data should provide you with the information that you need, but a "quick and dirty" cruise level is approximately 1000 feet for each 10 miles of distance, up to the optimum level.
For Example, if you are planning a 310 nm flight, 31000 feet would be a good "rough" level to begin examining wind gradient etc. to refine it further.
That works pretty well in ISA+15 environments for aircraft in the B757 league, but beware, all aircraft are different!!!!!
Regards and Welcome,
Old Smokey