My understanding is that FTP will typically be somewhere between 72% and 78% of final minute power of a ramp test. The 75% usually used is in the middle of the range, but will clearly be out by a few watts for many. With a ramp test last minute ave of 360W, for example, FTP is very likely somewhere between 260-280W.
If you really want to find out, take whatever figure you think your FTP is and see how long you can hold it for. If you manage around 40 minutes its about right…