Sparki angle correction

Hey everyone,

We have a project with friends using Sparki.
We are trying to make Sparki traversing an empty room avoiding wall.
Our code is good, Sparki traverses it. But we are trying to get the position back.

And we notice an error when he turns with the function sparki.moveRight or sparki.moveLeft. In fact, in the code we put sparki.moveRight(90) or sparki.moveLeft(90) and in reality it turns 93° or 87°, we would like him to be more accurate and do exactly 90°. I think maybe there might be a problem with the stepper.

We looked into sparki.cpp, and we were not able to find how to get him more precised.
We take a picture of what it should have done (orange) and what he really do using the pen(black).

If someone had the same problem and found a solution, we will be glad to hear about it.

Cheers guys