You simply cannot make an assumption without a tremendous amount of data that a multiplier of 1.07 is in anyway accurate for anyone else but you.
Even if it is true that there is a multiplier between indoor and outdoor FTP, which I do not believe to begin with.
Unless you are using the same power meter inside and out, you have a few percent on the trainer power meter and bike power meter. Even if YOU are using a crank based power meter, where you are getting data from the same device inside and out. Others are not, and others are using different power meters.
So to come up with some factor like this you would need high quality measurement equipment that you have many subjects come and use the same equipment so that you take devices out of the equation and leave it entirely up to the human generated power.
I’d also suggest that there are many environmental factors that can come into play. Such as fans, inside temperature and humidity vs outside temperature and humidity, wind, sun etc…
Also, outside if you are riding with friends and you get pushed mentally more to hold onto them. While inside you don’t have the mental toughness to push yourself on the trainer. You can find yourself finding that your ‘FTP’ seems different, when it is not different at all.