This is a bit of a bête noir of mine.
My company was training newbies to fly PAPIs to touchdown-and then wondering why we had so many long landings! Hardly surprising when so much of the TDZ was being wasted.
As mentioned, screen height at the beginning of the TDZ is 50 feet AGL, which in my cat C aircraft pretty much means that when the white line passes directly beneath my gonads (other genital organs are available), the RadAlt should be saying ‘50….’. Usually, at this point the PAPIs show three reds if set to 3 degrees.
So, as Capt Bloggs says, your trainer is erroneous in his assessment if you were in the correct position. I would clarify prior to your next training flight precisely how the trainer expects you to approach this.