Note: /walk has links to all recommends mentioned below (treadmills, mat, shoes, etc).
You've decided to use an under-desk treadmill / walking pad while working. You'll use this any amount of an 8-hour work day, and it's possible you'll do the full 8 (I've done it). Your speed will range between 1-3 miles per hour; and by week's end, you've put it more mileage than any sane exercise regime (up to 960 miles). So you really need to get the ergonomics right.
3% was decided as the optimal incline for walking / running (1, 2). The very slight upward climb reduces the distance to impact (reducing the impact), reduces hyper-extension, and stabilizes the right muscles around the knees. This makes choosing a treadmill easy, since not all offer incline.
Some worry about lower back strain as a trade-off, but I've never experienced lower back pain in either regime. Your back and posture are already improved by walking over sitting, so any concern here is negligible compared to knee health.
A 3% incline adds 20-30% more calorie expenditure for the same speed. This means you can set the speed to 20% less than you'd normally walk for the same benefits, meaning you're reclaiming stability at the desk (mouse and keyboard precision). Or you can walk at the same speed as before and lose more weight (I do the latter).
Two things: (1) motor life; (2) walking pad impact. (1) The treadmill's motor expenditure and wear is based on speed alone. So if you reduce speed by 20% for the same human effort (above), you'll reduce motor load by 20%. (2) The incline reduces the distance to impact, meaning there's less of an impact on the walking pad when foot touches down. This improves health on knees, and improves health on the walking pad of the treadmill. My mid-tier & premium recommends offer incline (set it to 3). My budget recommend does not; prop it with a 2x4 (or better yet, upgrade).
Now that you know all this, take the same knowledge to the gym any time you run on a standard treadmill. 3% should be your fixed incline, no more no less.
My budget & premium recommends also provide mechanized shock-absorption. I also recommend a treadmill mat, primarily to protect your floors, but doubling as added shock absorption. And of course, a good pair of walking shoes. With these four powers combined - 3% incline, treadmill shock absorption, a treadmill mat, and good shoes - you'll protect your knees significantly.
I trust the sports medicine expert claims, but I've also experimented with this over 4.5 years. I have bad knees myself. My first treadmills - for 2 years - were flat. I have knee pain, and it worsened noticeably during that time, thus sparking my research. I switched to 3% for the next 2 years and my knee pain went to 0. My newest treadmill goes to 9%, and I got greedy with the extra caloric burn for 2 months. The pain came right back. So I went back to 3%, and bingo within weeks. So there is a "too much", though flat is worse. My knees tell me everything I need to know and fast.