Howdy Northland: Welcome aboard. You asked about the 'right' way to span a medieval crossbow. If you are spanning with foot in the stirrup and place the trigger BESIDE your leg (not against it) that's probably the best way of making sure the trigger in a simple roller-lock bow engages deeply in the sear. Gravity helps make sure your trigger is engaged. Indeed I suspect many medieval bows had no trigger-spring at all. But yes, there's a possibility of hitting the trigger with your knee.
If you use a belt-hook, you'll want to put the top of the stock against your belly, with the trigger away from you. That shows up in lots of medieval illustrations.
If you use a wippe lever, put the butt on the ground and hold the stirrup, with the top of the bow (track) facing you. With a gafa (goatsfoot) lever, most people do as with a wippe, but there's an Italian model of the gafa that rests the lever against the upper thigh and you manipulate the crossbow stock rather than the lever. So everything works upside down (Illustrated in Harmuth's "Die Armbrust".
If you use a cranequin, the top of the stock will be upper most, but you may have either the butt or the fore-end (usually there's a little hanging-ring rather than a stirrup at the front) against your leg or belly, depending on the type of cranequin you're using. I have seen some illus of Spanish cranequins being spanned with butt against the body and the bow pointing out ahead of the shooter, more or less level.
With the big English Windlass, one puts the stirrup on the ground, and the winder fits on the tail of the stock. You could do that with the top of the bolt-track facing either toward or away from your body.
Too much information overload?
Okay, the simple answer is: I recommend you stand on the stirrup and put the trigger beside your leg, with the top of the bow facing away. When the lock is set, put your fingers beneath the long tickler-trigger to make sure it isn't hit somehow in the loading process. Geezer