Formulas like Karvonens are at best correct for 30% of the population -that means 7 out of 10 people will not be able to use the "age predicted" formulas effectively. Like have you seen those older riders that the formulas say should have a 145 BPM max and they can work all day at 165? Its like saying "Hey fit person follow this chart and become less fit!" Although as we get older your maxium achieveable HR will go down but not exactly on your birthday. MHR is based on one- your age and two your genetics and is special to you. Ever had days where your 160 bpm seemed very easy like you are pedaling without a chain and then the days when it seemed that your heart is coming out your chest at that same number? There are many factors that can make you go nuts if you stick yourself with a number. Things like sleep, nutrician, stress, hydration, how you worked out the day before are all factors to what the heart rate says about your intensity. The real problem is that HR is not a very stable way of telling what your intensity is. There is an area called deflection point where linearly your intensity will not progress with the bpm of your HR there is a point where as you get closer to your max the heart says "push all you want I can not give that much more." ever been there? Ever been in a state where its difficult to breath and you think "OK I'll sprint and just go for it" and you look at that number and go hard and when you have winded yourself and look back down you have only gained a couple of beats? Think about this. If your at a scary movie and all of a sudden a creature goes boo and your HR goes way up but you have no vienous return to be considered any sort of intensity other than a fight or flight response. one more way is to think about cardiac fade. Over time on an moderate endurance ride you lose beats per minute even though you feel the same amount of intensity.
A heart rate monitor is an awesome tool but learning what that number is telling you is the hard part. I suggest checking it and graphing it over time to get an idea of a area of numbers that will tell you when you are in the correct training zone. And as far as being able to say "I have become more fit" that happens when you see HR numbers that you can hold for longer or just feel more comfortable doing.
Velocity