Can I humbly suggest this. There's no exact way (?) to replicate the trajectory of your No1 rifle backsight with commercial or handloads. So what I suggest you do is this. Start with TWO known datums. You have the first - the backsight. Now beg borrow or steal 25 or so rounds of .303" UKicon Military Mk7. Use this to zero the rifle at 25 yds using the EMER zeroing spec. That way you know that your rifle is correctly zeroed and will be through the ranges that you'll be using on your backsight/rifle.

Now get some handload info and work up a load that is close - or as recommended by your pals. You adjust your hand loads to the rifle and sight. Not the other way around