Bad practice is worse than no practice.
You could always take one of the educated or theoretical guesses as to what your vehicle's rollout time would be, but what if that figure is wrong?
Bad practice.
Here is the most effective way I have found to get started with your practice tree.
Set it on delay box mode, with an even 1 second of delay, and ZERO rollout, practice hitting the top bulb, and using your button in whatever way is most comfortable and repeatable for you, or in the case that you already are using a button, as close to as it is in your car as possible.
You will be red every time, most folks will fall into a range of .170 to .220
Hit the top bulb until you have found a steady, consistent Driver Reaction Time......
I know it sucks coming up red every time, but bear with it......
Now say after some practice you arrive at the conclusion that your DRT is about .200, and you can fire up that practice tree anytime, and hit it right at .200 Most folks will have a natural variation of .005 to .010 plus or minus of the mean average DRT.
Now take the delay out of the box, still zero rollout, and hit the bottom bulb until your bottom bulb DRT is the same as your top bulb DRT.
At this point, you will have enough training for your eyes, because that is all we are doing is training your eye/brain reaction, your first handfull of time slips will reveal your car's true rollout time.
You will know whether or not you hit the bulb correctly every pass.
If you missed it, don't use that R/T for your rollout, only use the ones where you felt right, subtract the true on track reaction time from your known DRT, and you have your true rollout.
If you repeat .550's over and over, and your known DRT is .200 then your vehicle's rollout time is .350
This method works regardless of whether you are a top bulber, bottom bulber, feets or buttons, yes, different actions will get different times, but your eye/brain is trained well enough to know whether you hit the bulb, or if something else varied.
I know for instance, that if I want to be exactly .020 slower, I jerk my entire hand off of the button straight back toward my chest, as opposed to what I found was my most consistent action, which is rolling my wrist 90 degrees, with my thumb extended and stiff laying on the button.
I have about a .007 plus or minus variation that I can't control, but set myself up for an .008 to .012 R/T, so that if I "miss it" fast, I'm .501 and if I miss it slow, I'm no worse than .519
The biggest benefit to bottom bulb feetsbraker types is that when you need to adjust your R/T at the track, you will know exactly what that 5 lbs of air in the tires does to your rollout, or what that one click in the front shocks is worth on the R/T, without guessing and chasing your tail all day long.
I can hit any tree, at any track twice and know exactly what adjustments to make to the box or the car to achieve the R/T's I want.
You'll still miss a bulb every once in a while, every human will, but at least you won't be driving down the track, or reading your time slip and wondering "What happened?"