I suggest using SPA for these, not Face Crop. Face Crop is really meant for school portraits . More for headshots. The further out the zoom, the less accurate it will be for thigh up, feet up, ect. Especially when using the chin tio top of subject algorithm, it can really throw off the scaling to the legs or feet because it is forcing the head and top of hair which varies into a certain space.
The use case you are trying to do with Face Crop is like trying to fit a square peg into a round hole. It really isn’t designed for this application like you are doing.
SPA has a lot more options for alignment for full body and 3/4. If the images have feet then cropping thigh up would be more accurate using alignment modes 2, 3 or 4 and scaling it so the red line to place the feet is off canvas to an exact location below the bottom of the canvas. Mode 4 would also work with sibling images because it doesn’t align to the face. You can setup a “template” with just the SPA alignment layers and no background layers so it is transparent. then save as png or tif and it will save single layer with transparency. This should give you more reliable results.
Here is a link to the alignment modes videos so you can decide which mode would work best.