Bull is right heart rate is 220 – age. HR work for endurance athletes is all fine and good when you have evidence of maximal oxygen uptake and anaerobic threshold limits so you can use heart rate as an estimate of these to train at the levels you want.
For you i think it is a waste of time, stick to distance and a time for that distance you know tests you out but doesn’t overly tire you and then try and beat it each week. You will know if you are going to hard and during the ride and if you are you should drop down a notch for the rest of the ride and adjust your training accordingly. Also do not go by their calorie burning levels they are an estimate and can be very wrong a lot of the time.
Your physical job in a hot shed will also increase your heart rate due to the fact you are working in a hot shed. The heat affects your heart rate and will make it drift upwards without a significant increase in energy burning.