Gallery Position/Row-number automaticallly

[PowerDapp2]

I use a gallery for temporary collection of production-articles. I need the cumulative sum per article started on the top of the gallery and end with the actual element.

I know how to setup a position no. but this is not automatically updated when sorting other way:
http://powerappsguide.com/blog/post/generating-row-numbers

position	article-no.	article-description	used qty	total sum per article	cumulative sum per article
1	123A	vanilla icecream	2	10	2
2	234B	strawberry icecream	1	1	1
3	123A	vanilla icecream	1	10	3
4	123A	vanilla icecream	4	10	7
5	123A	vanilla icecream	3	10	10

the position number/id should be always the number you can see on the screen and it should not matter how to sort the rows.

gallery asc-sorted by 'used qty' and automatically actualized position:

position	article-no.	article-description	used qty	total sum per article	cumulative sum per article
1	234B	strawberry icecream	1	1	2
2	123A	vanilla icecream	1	10	1
3	123A	vanilla icecream	2	10	3
4	123A	vanilla icecream	3	10	6
5	123A	vanilla icecream	4	10	10

and same desc-sorted by 'used qty' and automatically actualized position:

position	article-no.	article-description	used qty	total sum per article	cumulative sum per article
1	123A	vanilla icecream	4	10	4
2	123A	vanilla icecream	3	10	7
3	123A	vanilla icecream	2	10	9
4	123A	vanilla icecream	1	10	10
5	234B	strawberry icecream	1	1	9