Hi again,
The test is for your Functional Threshold, which is:
(quote Cycling Weekly)

"Simply the best effort you are able to hold for an hour withour stopping. It is usually measured as power output in watts, but can also be tied to average speed or heart rate."


Whish is what the formulae posted a few above is for. Working out your average FT heart rate (FTHR)
The number can vary around 10% either way.

Hope this is usful to you all