Top sirloin grill temperature is a little nuanced.
The best way to do it is to have two levels of heat: You want one side of the grill to be at around medium-high for searing and browning and the other at a medium-low level for cooking the meat through.
If you're using propane, simply light two sections, one for each of the temperatures.
If you're using charcoal, you can accomplish this by putting most of your coals on one side of the grill and leaving a few on the other side.
Start by putting your steak over the medium-high heat and searing it for a good three to four minutes on each side.
After that, you can move it over to the medium-low side until it reaches your desired doneness.
Be sure to use thongs as using a fork will cause all the delicious juices to run out of your steak.
The best way to check for doneness is to go by firmness.
A good trick for this is to place your index finger to the tip of your thumb, forming a circle with your thumb and finger.
Now use your other hand to feel the base of your thumb.
This is what a rare cut would feel like.
Now use your middle finger and thumb to form the circle.
This is what medium-rare would feel like.
Moving down your fingers, your pinky and thumb would be a well-done steak.
It takes a bit of practice but using this method, you'll soon be able to tell the doneness of a steak by simply feeling it directly.