I gauged my HRmax on a steep climb that I am barely able to summit cadence 40rpm standing and muscling the bike.
It takes me about a minute but I am already warmed up and revved up for the approach to the hill. I'm sufficiently rested and caffeinated, and it's hot weather.
I assume that my observed max heartrate is a few beats slower than a true max.
My HR runs a little slower in cold conditions.

I'm not locked in to certain percentages or zones. My HRmax is probably 187bpm. I know that if I average 160bpm or higher, I am going to fall apart after about 2-1/2 hours riding. 155 avg is about right for a 60-90 minute ride. If I want a recovery ride, I better keep HR under 135. If I can't get HR under 140 during ride breaks, I need to find shade and get more fluids ASAP and possibly sag out to avoid a seizure. On shorter time trials I need to keep around 168-173 until the last mile.

To me, the important factor in choosing a HR monitor:
* Ability to easily upload the stats to a PC and then to some online sites (strava, my company's wellness website)
* Good battery life both in sensors and in computer.
* Ability to transmit underwater for swim workouts
* Ability to attach unit to either wrist or bike handlebars
* Ability to talk to gym equipment
* Open-format protocols, not proprietary -- in both transmitter and bike sensors.
* Use with my Garmin Ant+
* Viewable when I am wearing heavy gloves and long sleeves. Handlebar mount prefered.
* Buttons positioned so I don't hit them when flexing my wrists
* Wristwatch unit metal must be stainless steel (allergic to nickel in other alloys)

I haven't found a single HRM that meets all these criteria.
So I am using a Polar RCX5 for the first part of the year to get wellness points; and for swimming.
I use the Garmin HR monitor after I've maxed out on wellness points.
I haven't had success wearing two HRM cheststraps at the same time. Inevitably they shift around and the leads lose their skin contact.

As far as the cheststraps, I have had very good success with the more current elastic straps with the thin rubbery backing under the contact snaps.
The "soft strap" previous Polar model needed wetting down and gel to get good contact.
The Polar cheststraps don't crease like the Garmins do, which has more to do with comfort than reliability.
Although it's not really obvious, Polar does have different length cheststraps. They aren't one-size-fits-all.