They work pretty good, but they just get you in the ballpark. You will find you still need to "fine tune" with a test strip and do the usual burning and dodging. The chart may say (for example) the best exposure is somewhere say between 15 and 20 seconds... so from there, you make a test strip that only covers that 15 to 20 range. It helps more with dense negtives for me, in case I initially grossly understimated the overall time.