That's not exactly how SOS chaining works. After 70 calls, the game starts rolling the shiny generator 4 times instead of once, and the Pokemon is shiny if any of those rolls succeed. The chance of each roll failing is about 1355/1356. The chance of the roll failing twice is 1355/1356 of 1355/1356, or 1355/1356 squared. Since it's being rolled 4 times, the chance that it's not shiny is (1355/1356)^4. Therefore, the chance that it is shiny is 1-(1355/1356)^4, which is much closer to 1/339 than it is to 1/341.