gun mph relates to how the bullet is deflected off of its natural path by dynamic natural phenomena (WIND!)
DNE (doesn't equate to)
target mph (though I believe to be a useless number - but Ill give it a shot).
So the beginning, target size, would deal with angular size of target lets say 0.5mRad or like 1.7; we want to end up with a value that tells us WHAT? Sounds like what you're after is MOE - how much slop can shooter get away with and still impact target. Answer will be in the form of a circle (overlaid onto whatever shape the steel is). Vertically (depending on shooting position) generally speaks to gravity miscalc or poor application of fundies. horizontal can also be fundies but will reflect mainly wind miscalc.
So from here Im guessing that there's going to be way too many possible answers to make a field-useable table. I also don't like the term 'target mph' is basically your confidence in each shot - so are you saying there's a scenario where you see a target and mil it then range it and look at [formula327] say "yea im NOT shooting that target because its too small (according to target mph)." To get that answer for comps is dumb because everyone is shooting all avail targets to get most points - yes K/TYL and for hunting if you see what your hunting for your choices are [shoot it now] or [wait/sneak closer]. Im not doing competing SWOT analyses here - these are all more intuition type of things.
Point is this one has no point.