transform.RotateAround in a specific amount of time?

I want to make a GameObject to rotate around a specific Vector3 in, let’s say, 1530ms. Every time I test my script, the object rotates around the vector at different time frames – for example, in 5 tests, while trying to achieve the timeframe of 800ms, the results were: 10138ms [my computer lagged], 827ms, 828ms, 779ms, 802ms. Everything is in a FixedUpdate. Here’s my code:

    void FixedUpdate()
    {
        transform.RotateAround(target, Vector3.up, 0.885f);
    }

The 0.885f was the “closest” float to achieve a result of 1530ms. (for testing I used a stopwatch from the System.Diagnostics namespace to measure when the object finished rotating)

EDIT: The Stopwatch from System.Diagnostics is actually affected by framerate, so it isn’t a good way to measure time. Anyway, I’ve ultimately given up, so I guess there isn’t a way to do RotateAround() in a specific timeframe…

transform.RotateAround() takes an angle as third parameter, so I assume you want the object to rotate by a certain angle in the given amount of time, right? If so, why haven’t you said anything about angles?
However, if so, then just divide the angle by the amount of time to get the step which will represent the angle to rotate in every period of time to get to the target angle in 1530ms in the end.

float angleToRotate = 90f;
float timeToRotate = 1530f;
float stepAngle;
void Start()
{
        stepAngle = angleToRotate / (timeToRotate / 1000f);
}
void FixedUpdate()
{
    transform.RotateAround(target, Vector3.up, stepAngle * Time.fixedDeltaTime);
 }

Or maybe you want simply rotate the object with some speed for the given amount of time and you don’t care about how far the object will rotate?
If so, then why wouldn’t you just use some timer?
Like:

float timeToRotate = 1530f;
float timer;
float someSpeed = 30f; // degrees per second
void FixedUpdate()
 {
     while (timer <= timeToRotate / 1000f) {
         transform.RotateAround(target, Vector3.up, someSpeed * Time.fixedDeltaTime);
         timer += Time.fixedDeltaTime;
      }
 }