What would be the maximum bullet weight diference relating the preparation of reloading?
In some places 0,5 grain difference is specified /+- absolute value is calculated/.
For example I weighted 40 grain Hornady V-Max .224 cal and the result was two groups: 39,8 grain and 39,9 grain. Can I consider them for being max precise? Can I mix them to one box? Thanks

a tenth of a grain difference will never be noticed, even by the most stringent competetive standards or the loads being shot by the best marksmen out there. For competetive shooting I would want them within 1/2 grain difference but for hunting or target shooting even several grains difference in bullet weight is ok and won't be noticed by even the best shooters.

personally 1/2% variation in bullet weight for a batch of bullets is about as much as I would accept.
if you have a variation of 7grains bullet weight in a batch you will have so much varried pressures from round to round that the groups would be more like pattering a shot gun.
with cast bullets i will allow a couple grains in heavy bullets 200grains and heavier.