Let's
N = p * 790 + q
then
c1 = 48 + q div 10
c2 = 48 + q mod 10 + 10 * p
so
p = (c2-48) div 10
q = (c2-48) mod 10 + (c1-48) * 10
test:
var
c1, c2: Byte;
n, p, q, t: Word;
begin
for t := 0 to 6229 do begin
n := t;
pCodeNumberToC1C2(n, c1, c2);
p := (c2-48) div 10;
q := (c2-48) mod 10 + (c1-48) * 10;
n := 790*p+q;
if n <> t then
Memo1.Lines.Add('Failed at ' + IntToStr(t))
end;
Final:
function C1C2ToCodeNumber(C1, C2: Byte): Word;
begin
Result := ((C2 - 48) div 10) * 790 + ((C2 - 48) mod 10 + (C1 - 48) * 10);
end;