Or if sphere is not what you need, square like distance between 2 points (where other could be map center) would go something like this

[pascal]function SquareDist(p1x,p1y,p2x,p2y: integer): integer;
var dx,dy: integer;
begin
dx:=abs(p2x-p1x);
dy:=abs(p2y-p1y);
if dx>dy then result:=dx
else result:=dy;
end;[/pascal]

Using this (should) result a
Code:
2 2 2 2 2
2 1 1 1 2
2 1 0 1 2
2 1 1 1 2
2 2 2 2 2
To have it inverted, just decrease result from highest value
(ie. final = 100-result for a 200x200 map)