mainoff is the nubmer of ticks that the changeable delay takes.
because its a delay, its unsigned making it in range: 0 - 0xFFFF

Now for mainoff ticks to pass till the overflow bit is set I need to fill the timer with its two compliment of mainoff and thats what I did.

The code is in a crtical section so it has to exectue fast...

Anyway in the end I used
temp = ~(mainoff);
temp++;

TL2 = temp;
TH2 = temp >> 8;
where temp is unsigned int...

which results in:
mov a,_mainoff
cpl a
mov r2,a
mov a,(_mainoff + 1)
cpl a
mov r3,a

inc r2
cjne r2,#0x00,00124$
inc r3
00124$:

mov _TL2,r2
mov ar2,r3
mov r3,#0x00
mov _TH2,r2


which could be hand optimized to

temp = ~(mainoff);
temp++;
_asm
mov _TL2,r2
mov _TH2,r3
_endasm;

but its less portable and only saves two commands...

Thank you all.