Originally Posted by noodles
Yeah, this, though you also want to have some idea what your max is so you can accurately define heart rate zones. Absolutely go for broke either running up a long-ass hill wearing a heart rate monitor, or on a spin bike, and go until you think you're going to pass out, puke, or suffocate. Wherever your HR maxes out on that effort, that's a pretty good estimation. Barring that, 220 minus age is a pretty standard way to ballpark it.
Calories used the way you are are probably fine, but all my personal trainer friends bitch about Fitbits because the methodology isn't especially accurate (not to sell them short, they do fine based on the data they have, but without a direct way of measuring output it's just a crude estimate) and it encourages people to think about how many calories they can take back in after
a workout, which is problematic if the estimation happens to be something like 10-20% too high (not uncommon). Time alone isn't great, because a 30 minute walk and a 30 minute threshold-zone run are going to have two VERY different impacts on your overall health and fitness. Time at a subjectively measured strenuously effort isn't bad, but again you have to be honest with yourself.
Really, the gold standard, if you care about tracking calories burned, is some sort of direct estimation of your energy output. If the spin bike you're on includes power metering, then awesome - that should get you pretty close, and as your heartrate tends to drop for an equivalent effort as you build fitness as your body is better able to handle the stress. But, that's uncommon and the serious spinners I know seem to care mostly about heart rate and cadence, so I don't imagine they're common.