Hello,
To answer this question, you have to understand the difference between incremental encoder and absolute encoder.
With incremental encoder, you have zero signal, and two other outputs of the encoder for instance. with these outputs you can know the direction and position refenced by the zero signal. i.e in your case your encoder is 360ppr. so if the HSC reads 135 pulse. that means that the shaft of the encoder has been moved by 135 deg. from its zero position.

Now If you have power off the counter module, and power on again, The read will be 0 and not 135. It will increment again until reaching to the true zero and then it will count in a correct way. So always the first revolution of he incremental encoder after power on is not presized.

Some of the machines can work in this way. i.e. if there no possibilty for mechanical damages in the machine if it has worked in this way

.
Some other machines can not work.

So you have to use Absolute encoder. It work in a different way. It have a rotary disc and it's not based on pulse increments. In each position of the encoder there's something like finger print for the photo detector. So every where you could know the position even if the power fails.

My recommendation. If you have to know the correct position after power on and the machine may deffected in the first cycle if you do not know that position, then use an absolute encoder with FM351 or you can use profibus-DP absolute encoders.

Hope I could helped you.