Forza, place an oven rack on the highest position it can go. Place your stone on top of that.
Measure from the top of the stone to the broiler.
Your oven and required set-up are likely different than mine, but some examples of things to consider for your specific situation:
Here are some quick shots of my oven...it is not set-up for pizza making in the pics.
Picture 1: The oven chamber. As you can see, my oven has the broiler on top. The broiling element has a pipe with holes in it where the flame comes out, with a metal deflector plate above this pipe on either side to help direct heat outwards and down.
Pitcure 2: Close-up of the broiling element. As you can see from the circled part of this photo, my particular oven has flame exit points which point downwards on the part of the broiling element which is closest to the pizza. If your oven is like this, you'll want to pay close attention to this area. In my oven, the flames tend to vary in intensity from this point, causing the flames to occasionally "lick" downwards too far and, at times, ignite any olive oil on the pizza! This is why I offset the stone in my oven when making pies.
Picture 3: My stone placed on the highest rack. From the center point of the broiling element to the top of the stone is exactly 2 inches. When I make pizza, I at times place cut quarry tiles under the stone to close the distance to 1.5 inches.
Now Scott123 may likely laugh at this because I am wrong, but for the all broiling technique I personally have had success with the thin stone you see pictured. I entirely encase my upper rack in aluminum foil and jam some in the sides and back to restrict air flow between the gaps where the shelf does not meet flush with the oven chamber. My hypothesis is that because this stone has 1/2" of clearance on the bottom because of the supports, enough hot air from the broiler gets under the stone as well, which allows the decreased thermal mass of this small stone to recover very quickly. Your bigger Fibrament stone may in fact be better and not require such tinkering.
Forza, another thing you want to do is pay attention to your broiler cycles and time to reach peak flame:
1. Turn on your broiler. Watch it. How long does it take from the time you hit the broil button to when the actual flame starts coming out? It takes a full 35-45 seconds for this to happen for me. Try this multiple times over a couple of days to get a good idea of the average time.
2. Once the flame is on, how long does it take for the flames to reach maximum heat? Without a thermometer, watch the flames. The dominant colors in the flames may change as the heat reaches maximum output. Pay attention to how long this takes to occur...use a stopwatch or kitchen timer. I've done this multiple times in my oven....on average, it takes my broiler 4-6 minutes to reach its maximum flame temperature once the flame comes on, which means that once I hit the broiler button, it takes 5-7 minutes before my flame is even at max temp. This is important to know. On an oven like mine, if someone just turned on the broiler for 10 minutes and put a pie in it, because of the delay time, the broiler would have only been at max heat for 3 to 5 minutes...not nearly enough time to get the stone hot enough for pizza making!
3. Does your broiler cycle...meaning, does it shut off if the oven gets too hot? If it does, how long does it shut off for? How long does it take to come back on? Use a watch or stopwatch function on your iPhone to time this.
These are examples of some things to think about, as "getting to know" your particular oven will be very helpful to help think about how to approach the oven set-up/workflow needed to heighten your chances of success.