Looking for suggestions...... 40' bridge rule

I have a trailer that is not marked in any way....

Where and how is the best way to make the 2 marks I need to identify the right locations for the 40' mark on a typical 53' trailer?

I heard there are different rules in different states.... is it 40' between kingpin and middle of FRONT axle, OR REAR axle (assuming that the trailer has 2 axles together)? I confuse easily and want a mark that can be seen in the mirror. Putting a mark(s) on the rail where the front pin would need to be for either rule might help when I have the luxury of a spare set of eyes to assist....

 

I hate math, I hate it, I hate it....

I currently use a modified axle stopper(Tandem Slide Stopper | Iowa80.com)... I inserted an eye bolt through the hole, with matching nuts on the inside and outside and secured those with thread lock...

I also use the "trucker slide calc" app to do the actual math... I measured the distance between pin holes(4"), input the steer weight, drive weight, and trailer weight; then I tell it what hole the front pin is currently in.. it tells me what adjustment (if any) are needed or possible.

 

I think that this would have been helpful as I started learning (I am not done yet)....

Here is how I use my tools:

Input my weights into the app. If it says to move my axles rearward(----->), say 5 holes, I count 6 from the 1st open hole behind the axle group and put my slide stop there. I then move my trailer until my slide stops, then reengage my pins. I then pull the trailer forward slightly until the pins engage. Then I can remove my slide stopper...

Then I reweigh and I am usually legal.