The usual purpose of sidescraping is to keep the weak armor behind some hardcover, in the case of IS-M vs. IS-M primarily the lower and upper front plate, and the angled piece above the tracks. As such, while not wrong in his analysis, the values that Nethaniel provided for the lower and upper frontplate would be basically irrelevant, because ideally you are exposing none of those spots to the enemy while sidescraping - you just go back far enough to clear the gun.
For the IS-M you'd have two sidescraping angles, either the gun points just over the forward tip of the track (still over the front of the tank!), a little less angled than you showed it in your first screenshot from tanks.gg. That would just leave the "sensitive" little angled piece of hull armor just above the frontal tracks exposed, while the side is still at autobounce angle. Or you angle like in your first screenshot, also hiding the sensitive angled piece of hull armor, but parts of the sidearmor are no longer auto-bounce.
Things change when your tank is fully exposed. In that case you might need to angle a little more to increase the effective armor of the frontplates, but without weakening the sidearmor too much. Tricky...
With that said, looking at your in-game screenshot I'd say you are overangling in the first case, as the gun is pointing further over the side than necessary. If the tank is fully exposed, it might actually be fairly okay against another IS-M - against tanks with more pen though, good luck....
... and meanwhile most of my rambling has been covered by others, and better as well....
Picking up on Bora_BOOMs point, it's not just about getting behind cover, but actually coming out of it to fire a snapshot, and doing so before the enemy himself makes it into cover. With that in mind, the "ideal" angle will work when you just stand there pre-aiming at a target/position, but it might not work for snapshotting enemies around corners.
Edited by WhoCares01, 17 May 2018 - 01:04 PM.