Don't sweat the percentages. Everyone loves to tell other people how to do UWorld, but the truth is you just have to do what works best for you. I know most people use UWorld for 8-10 weeks leading up to their exam, but my med school didn't have dedicated so I started UWorld about 4 months prior to my exam.
For step 1 (which I assume you're asking about), the percentages tend to get better over time. Be sure to study your incorrects and make flashcards from them. The general recommendation is to use UWorld as a way to test your knowledge. Your percentages will tend to reflect your knowledge-base and test-taking ability, but they aren't predictive by any means! Focus on the questions you get wrong, spend more time reading about those topics and making sure you don't get them wrong again. Every question you get wrong on UWorld is an opportunity to never get that question wrong on the actual test!! For reference, I got a 253 on my step 1.
For shelf exams and step 2, I used UWorld as my primary learning tool. You just don't have time to set aside all the questions leading up to the shelf and the explanations are too good not to read right away. So, the percentages don't matter much at all during clerkships. I got a 275 on my step 2 CK.
Your mileage may vary, so figure out what works best for you! The best piece of advice I ever got in medical school was "don't listen to any advice, including this advice." Cheers! Lmk if you have any questions.
- Eric, Elite Medical Prep Tutor