Stab trim
The formula comes from the linear relation ship of the two values... If you look on the QRH there is a small " nomo graph " that starts at 35% etc. So if you subtract the Mac from 35 and divide by 2 it lines up the two variables.
You set stab NU etc in the MCDU performance and this is part of the trigger for stab not set warnings etc.
I recall that in non auto trim setting 330s this is how you set the trim whereas in the auto trim type , it sets it after the first engine start to the ECAM value of CoG.
That is why some companies seem to check and state the stab with reference to % , and others state the stab value.
Either way , the value should be set accurately by the PNF. I think it is just to ensure the MCDU Value is close enough to the actual set value to prevent the config warning if >2%? But I have been known to be wrong !