Nominally the chain stay angle is the angle between the chain stay's centerline and the seatpost.
The Seatpost angle is between the seatpost and the horizontal.
However there is some slight discrepancy between the two
Chain stay angle (Actual) This uses the centerline of the physical chainstay
Chain stay angle (Rear axle to BB) This uses a straight line ...
OK, they work with the same shifters and the same chain, but they are NOT all compatible with your frame. FD-M3100-M is front pull while your FD is down pull (at least it is mounted like that).
Most FDs are dual pull, that means both top pull and down pull is possible.
Altus FD-M2000-DS3 FD-M2000-DS6 (down swing)
Altus FD-M2000-TS3 FD-M2000-TS6 (top swing)...