How to evenly portion chapati dough?
Even portions means same weight: a scale is the tool of choice here.

Bakers that want equal sized products just weigh the individual portions, then proceed from there. 

The shaping / rolling of chapatis was already discussed in this Seasoned Advice post. Or you could use a press1 to shape them.

If you are thinking of a larger scale of production, there are hydraulic chapati presses1 or even (semi-)automatic machines1 on the market, that shape chapatis of the same thickness out of equal sized (= weighed!) portions.



1 No affiliation or recommendation, just random samples from the Internet!