Oak - 47 while perhaps being hight for a non-athlete, doesn't seem unreasonable for you - it's in the pretty normal range for a female cyclist.

I've had it done twice. Once very professionally by a coach/pt on stationary trainer with my own bike and once for a research study I signed up for - that time it was on a treadmill and was an estimate. The first time was real - I was about to the point of wanting to puke when it was over. The second one was pretty inaccurate, I barely got past a fast walk before I was stopped… and the chart the machine produced didn't have a wide enough range.

For me the test pretty well confirmed what I knew about my hr zones, but did allow me to set power zones.