There is no simple formula, since both examples you provided could tell you different information about your current fitness level. You could probably run much longer at a HR of 160 than you could at 185. At 160, you may have the fitness to run a marathon. At 185, you may not be able to run much farther than a 5k. Is one "better" than the other? No. It just depends on your goals as an athlete. Due to that fact, it's hard to derive one formula to "normalize" your results.
For a great primer on this topic, you may want to read this article on training zones from Greg McMillan McMillan Running - Six Steps to Success. In very simple terms, he explains the physiological changes that occur as you run at various paces.
If you want to use heart rate to measure improvements in your fitness, keep two of the three variables (pace, distance or time, and HR) constant. For example, set a target heart rate (let's say 160) and fixed distance (let's say 5k). Time how long it takes to run the 5k while maintaining a heart rate below 160. Perform this same test weekly or monthly. If your time improves while your heart rate and distance remain the same, you will know that your fitness is improving.
There's a version of this test called the MAF (Maximal Aerobic Fitness) test that's somewhat popular with endurance athletes. You can learn more from this article: Phil Maffetone - MAF Test