edit: nvm, i re-looked at your math, and i originally mistook it as being the culprit, could your provide a better detail of what's the problem your seeing?

this is a simple parts over base = rate over percent problem.

your part is your player position which is (PlayerPos+UpperOffset)(which in your case is 4096x4096)
your base is the map's dimensions which is 8096x8096

your rate is x which you need to solve for
and your percent is the target size, which is 1024 in your current case:

so it'd look like this:

PlayerPos    x
------------ = ---------
HL2MapSize    TargetImageSize

solving for x is simply cross multiply

so PlayerPos*TargetImageSize/HL2MapSize = x

this is a simple parts over base = rate over percent problem.

your part is your player position which is (PlayerPos+UpperOffset)(which in your case is 4096x4096)
your base is the map's dimensions which is 8096x8096

your rate is x which you need to solve for
and your percent is the target size, which is 1024 in your current case:

so it'd look like this:

PlayerPos    x
------------     = ---------
HL2MapSize    TargetImageSize

solving for x is simply cross multiply

so PlayerPos*TargetImageSize/HL2MapSize = x