Seems like a bad idea -- over training, under recovering, exercising at the wrong intensity at the wrong time in the workout, etc. It's fine to have a "free ride" instead of a training day. You could play with intensity more in that setting. But, you shouldn't make every day a play day if you want to get better. I've learned that the hard way and I'm still learning it every day.
Getting an accurate FTP figure is an art in itself. It's more of a guess than anything else usually. It's an estimate, rather than a fixed number.
If you're finding that you can do the intervals easily at your target wattage then you can always increase the target wattage figures for next time and see how it goes. Some intervals are meant to be done at less than 100% effort though so it really depends upon the session that you're doing.
If the new target proves too hard and you're falling short you can then reduce the target wattage a little until it's achievable again.
I pay attention to intensity factor on my long rides. When I start to see it consistently creep up chances are that my FTP has risen.
Testing is difficult to do and getting an accurate test is easier said then done. Personally I only test four or five times a season, often less. Partly because more often then not my FTP estimates by looking at training data is pretty close.
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