Measuring my max heart rate

I've bought this book about training for sportives, and its all based on heart rate zones, fair enough.
It says to measure your maximum heart rate on a turbo trainer slowly cranking up the effort over 15 minutes until you can crank up no more.
If I don't want to setup my turbo trainer, would it be a safe enough assumption that the maximum heart rate I acheived go up a bloody steep climb midway through a sportive would end up the same as this trubo trainer practice?
It says to measure your maximum heart rate on a turbo trainer slowly cranking up the effort over 15 minutes until you can crank up no more.
If I don't want to setup my turbo trainer, would it be a safe enough assumption that the maximum heart rate I acheived go up a bloody steep climb midway through a sportive would end up the same as this trubo trainer practice?
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At real max HR, you can lose control of the bike.
Rule of thumb, add 5bpm to the highest number you've ever seen.
2009 here I come..... after a nice piece of cake.