The question which bothers me is how I should calculate my salt additions. I know the method of calculating particular salt addition, but the question is should I divide it by proportion for mash water and sparge water?
It is a good question, since there isn't truly a right answer.
You could add all the salts for the batch in the mashing water, add them all to the kettle after mashing, or split them between the mashing and sparging water. But there are differing impacts in each case. The good thing is that most of the ionic content that you're targeting with your salt additions will make it into the beer. But there are definitely mashing impacts that need to be addressed and accommodated in any of the cases. Add all to the mashing water:
Most helpful when you are targeting a low ionic content in your wort, like a Pilsen water. Adding all the salts to the mashing water can help drive up the calcium content which reduces mashing pH and can increase the calcium content above 40 ppm which helps remove oxalate (beerstone potential) from the wort. Add all to the kettle:
Some think that this helps avoid ionic losses in the mash, but it doesn't. It just delays the precipitation reactions until the wort is in the kettle. Adding all salts to the kettle can help avoid an overly low mashing pH by keeping the calcium and magnesium out of the mashing water. But this trick doesn't help too much since the kettle wort pH will drop when those salts are added in the kettle. For most brewing, utilizing the beneficial pH lowering effect of including the calcium and magnesium in the mashing water, is preferrable and this is the reason I can't recommend this approach in typical brewing situations.Add proportions of salts to mashing and sparging water:
This works well when your targeted calcium content is already above 40 ppm and there is little worry that your wort won't precipitate its oxalate. This method provides some pH reduction in the mash from calcium and magnesium additions and that may reduce the amount of acid that would be needed in the mash.
Take your pick, but understand the consequences.