Factors to improve score in GMAT

Hi I have a doubt about the Gmat test and my potentional performance when taking the real exam. I took a Gmat Mock (Cat) for the first time, I scored in a range of 550-650. Nevertheless, I practiced with intermidiate level questions (550-650) and hard level questions (700-800) and the percentages of correct answers for each level are 40-50% and 30-40% respectively within an alloted time of 1 min for SC, 2 for CR, 1.5 for RC questions, 2 for PS and 2 for DS.I think that if one cannot answer a 700-800 question correct that person may not have reached that level. The doubt is, should I focus on hard questions only or try to combine both levels? Do you think I reached the range of level of 700-800 or how would you suggest I can measure the correct level?

Any advices of Gmat instructors are welcome and thanks for your support