For an engineer to be able to specify a maximum, especially so specifically, then you can only assume that this is based on entering the screw without a mount (otherwise the mount thickness would need to be known or of a given fixed standard)
To answer the OP question and basing the calculation on the likely thickness of a mount being 2-3mm … I think it is safe to say that a maximum screw thread protrusion value beyond the mount then entering the DT should be about 4mm … anything more than that and you are possibly gambling with issues … my hunch is that you’ll get sufficient purchase with at least 3mm and you’d be safe with 4mm, more than that will not be of any mechanical benefit
Specifying a maximum screw length is perhaps useful if you want to absolutely guarantee that under the worst scenario (screw stored directly on DT) you’d still be safe, but it doesn’t help those trying to work with thicker vesa mounts or washers or custom mounts get the important practical info which is twofold
see image below
- what is the minimum value of x (to get adequate purchase)
- what is the maximum value of x (to prevent damage)
In terms of specifying useful info, this is all that matters, but it’s reliant on people being able to verify this accurately
My recommendation for anyone wanting a rough working measure would be that the screw depth penetrating your mount offered up to the DT should be no longer than it’s broad … so 3-4mm should be fine … anything less risks you stripping the thread if you over tighten
This is my thought only, always check through official channels or very gently check with a cocktail stick (or slither of plastic) how much depth you have to play with … when the unit is powered off and make it less than that in any case to account for flexure
X=4mm max is a safe common sense assumption imho
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