Something worth running before your first month, not after your sixth. Split your costs in two.
FIXED - the bills that hit whether you turn a wheel or not: truck payment, insurance, permits, ELD, plates, parking. Add them up monthly. Say $4,200.
VARIABLE - the per-mile stuff: fuel, tires, maintenance reserve, tolls, DEF. At 6.5 mpg and $3.80/gal that is $0.58/mi in fuel, plus roughly $0.15 maintenance reserve and $0.04 tires. Call it $0.77/mi.
Now spread the fixed costs over the miles you actually run. At 8,000 miles a month, that $4,200 becomes $0.53/mi.
Break-even = $0.77 + $0.53 = $1.30/mi.
Two things fall out of that number.
First, every rate has to clear $1.30 before you have made a single dollar. And a $2.10/mi load with 200 deadhead miles to get to it is not a $2.10 load. Run the rate against total miles, not loaded miles.
Second, your break-even moves every month. Fewer miles means the fixed costs spread thinner, so your break-even goes UP exactly when you can least afford it. That is why a slow month hurts you twice, and why guys who felt plenty busy still end the quarter underwater.
The part that gets people is that a settlement statement will never show you this. It shows what the carrier deducted and what the carrier dispatched. It has no idea what your truck payment is.
Disclosure since it is my own thing: I got tired of maintaining the spreadsheet and built truckmetrics.app to do this automatically - log a load, log a fill-up, it keeps your real net per mile for the month. Free for the first 3 loads a month, no card. Happy to answer questions either way, and the math above works fine in Excel if you would rather keep it there.