We are very individual. 40 grams of carbs might be just fine for you. You can always try a few weeks and see what happens. We recommend 20 grams because it tends to work for almost everyone. But even then, as Susan said, there are some that have to go lower. Several people on this forum use “20 total carbs” instead of “net carbs” for this reason.
For myself, I was very lazy keto at the start and didn’t track anything. I lost about a third of my total eventual losses just winging it by eating the ‘right’ foods but without any counting. Eventually, I stalled out and took stock with some careful tracking of what I was eating. It turned out I’d been having closer to 50 grams a day than 20. Tightening this up got me losing again at that point.
So, if you really want to start at 40 grams you may be fine there and lose weight at least for a while.
I hope this helps!