Tracking expenses and keeping expenses in line is key to the bottom line profit. Remember, gross profit pays expenses and what's left over is net profit...the bottom line. To truly understand what your budget should be takes an understanding of the Profit/Loss statement. But, I hesitate to go into detail, only because there are so many key performance numbers to consider, (labor profit, part profit, sublet profit, etc.) and also you need to look at your debt; which is not on your P/L, but on your balance sheet.
I am not here to pitch what I do for Elite, and never have, so don't think I am trying to sell anything....but I spend an entire day teaching all this at Fly with the Eagles Course. I only tell you this, because it may be difficult to explain in a post.
One very good thing, you are asking the right questions! And you absolutely should have a written budget.
Speak to your accountant too.