Max Heart Rate

No formula is correct for everyone.   The only way to accurately find your max is to reach it, which is potentially dangerous (medical supervision recommended).   But your MHR is at least 177.   177 is probably a good figure to stick with, and calculate your zones from that.  If you ever happen to exceed it, adjust accordingly.

I use a polar m400 watch and have got a handlebar mount. Brillaint training tool and I totally agree about the usefulness on heart rate monitors. You can customise your heart rate zones, I usually ride around zone 4, 175-180 for around 1 and last couple of miles go 190s. Interesting though when I'm out running I can push Upto 205, didn't actually realise this was possible before. But anyway, I think people can get too focused and beat them selves up about times, on a bad day you will be slower but your HR monitor can tell you exactly how hard you have worked. You can also test recovery by taking a resting hear my rate on a morning just as you get out of bed, if it's higher you know you are still recovering. 

There are different ways of calculating it but it's primarily used to allow you to define your heart rate 'zones' that are useful to

1. use as a base for training sessions (this explains the basics: http://www.bikeradar.com/gear/article/heart-rate-monitor-training-for-cy...)

2. have an idea of how much you're pushing (eg. if you're out on a particularly nasty club ride but are only hovering around zone 2, you can probably go all day; if you're in zone 5 you need to back off)

3. track fitness/form against your previous benchmarks.

I use my HRM even for commutes, which I use as training or recovery where I can; I know my usual ride into a 20mph headwind will be a tough session, but I can use my heart rate to tell me a bit more about exactly how tough I've found it and whether I need to take the next day especially easy. Sites like strava and training peaks give you a bit more graphics and info on that.

Then, if you want to get even more spendy/serious/analytical, you can throw in power meters too...