Greetings...
When you design several layers of metal or poly, and you must change the direction, you must use 135 (or 45) degrees. This is better because you must keep soft changes of direction, in order to get a constant impedancie across the line. Applying closest corners (e.g: 90 degrees), the effects of corner difraction get more importance. Also, you must think in construction. Typically, IC factories only can work with this angles.
In order to calculate impedance, sectors above the 145° corner must be multiply its constants by 1/3.