I don't know if I recommend you trying to make the tool you describe yourself, but it won't cost much to try, so go ahead.
As to WHY bothering to know the length at which point the bullet hits the lands... because, as you will discover if you study more about reloading and experiment yourself, different bullets and rifles like different specs. No one rifle is the same as another. Sometimes a rifle prefers a certain bulletin to be seated against the lands, some like to be a bit off the lands, or a bit more yet. If you determine that it prefers to be a certain distance from the lands, then you want to be able to duplicate that later. Also, in time, the throat erodes and therefore the start of the lands will move further down the barrel. As it does so, you may find it prefers the bullet to be seated longer and longer so as to keep it's specific proximity to the lands. Without measuring and experimenting, you'll never know that.
However, if you are not particularly interested in handloading for the sake of the challenge of finding optimal accuracy, it isn't really necessary to do this. Load 'em and shoot 'em. Have fun.
One caution: Be careful not to seat the bullet so long as to actually bury the bullet into the lands. Doing so can cause excessively high pressure and be dangerous.
One cannot determine whether your bullets are seated correctly based upon the length a similarly weighted bullet is seated in a factory round. Weight of the bullet has nothing to do with the shape of the bullet or the spot it hits the lands.
I'd suggest you study your reloading manuals closely. They all address the issue fairly well for general purposes and safety.
Finally, OACL does not really have much to do with anything other than determining whether the cartridge will fit in your magazine.
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