It doesn't suck too bad. But, you should be aware of a few things.
It's normal for the bullet to stick and it's easy enough to knock it back out with a cleaning rod. But, you have to use a little "feel" and avoid jamming it hard into the lands. Even though it sticks, you should still get pretty consistent results.
While measuring, you want firm pressure foreward on the outer shaft to keep the shoulder of the modified case against the shoulder in your chamber and a little less pressure on the inner shaft against the base of the bullet to have the ogive just touching the lands.
You should easily be able to get within .001-.002" consistency. The precise number you come up with isn't the point because it's only an arbitrary starting point for a load workup for a given rifle/bullet combination. You may start off wanting your bullet .010" off the lands. But, your final load workup may give you best accuracy at .140" off the lands, or .015" into the lands.
Similarly, your throat may errode over time and you may need to seat your bullets out farther to offset that.
Don't forget that you have to re-measure for every bullet make/model that you want to load because they all have different shaped ogives.
You really need to measure the OAL of the bullet in the modified case with a comparator such as the one made by Hornady in order to measure the length from the ogive rather than the meplat. Where the ogive contacts the lands is what counts in seating depth and tends to be much more consistent than the meplat.
Even if you measure consistently using the OAL gauge and a comparator and allow for exactly .010" jump according to your measurements.... you aren't necessarily a precise .010" jump from the lands because the datum line formed by the comparator insert for your caliber isn't necessarily the precise inside diameter of the lands in your bore. ...just about every rifle bore has a slightly different inside diameter and geometry of the lands and grooves.
That doesn't matter because you're only looking for a valid starting point for your load workup.
However, you do need to take into account headspace. And, here's why...
The HNL modified cases aren't exactly fireformed in your chamber. I have measured some to actually be shorter in length than SAAMI tolerances.
Suppose you have an extra short modified case for your OAL gauge and you measure from the case head to the ogive and calculate .000" jump. For illustrative purposes, let's say the modified case is .250" shorter from the shoulder to the head than your actual once-fired, neck-sized case. When you seat your bullet in the actual case with the measured OAL, you will end up with .250" jump rather than .000". This is because they are both the same length from case head to ogive. But, the modified case was .250" shorter from case head to shoulder.
As such, you also need the Hornady HNL head space comparator inserts and you should measure your modified case from the datum line on the shoulder to the case head and do the same for a fire formed case and add in the difference for your OAL.
If it sounds confusing, it's not really. But, I may not have explained it well. So, draw yourself some pictures and take some measurements and you may come to those same conclusions.
If you still aren't convinced that this is the best method, the you can simply create a dumby round with no powder or primer. Use light neck tension and seat the bullet too long. color the ogive with a sharpie and gently close it in the chamber. If it doesn't close, then seat it deeper and repeat until it just closes and doesn't leave marks on the ogive. Then measure the OAL to the ogive, and/or use the dumby round to adjust your seating die.
Sinclair and others make various tools that accomplish the same thing. They all have their drawbacks.
Hope this helps.
-- richard
