Scott,
There a couple misconceptions in your original message. You will find that in some pens the fountain pen reservoir might be the critical dimension for the depth of the blind hole in a closed end pen if you are using the Schmidt reservoir, and that it might be necessary to have a short plug or a longer spring behind the rollerball cartridge if it is installed in the same pen.
It is always a good idea to lay out the internal parts of the pen, as they would be assembled, before starting on any pen that is a modification of the kit. This will give me the dimensions that are specific to the kit and parts I am using, and I will always have a reference available when I forget or can't find the dimensions that I may not have remembered to write down.
Here is a photo of the assembled internal parts for one of my early clased-end pens in the various forms that I would use.
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The top pen is the first closed-end pen that I made about 5 years ago. Below that is the stock Ameroclassic made from the kit. The one in the middle is a fountain pen with a spare cartridge. The bottom two are a rollerball and the Schmidt fountain pen reservoir.
I am now doing the same thing with the Baron, and with similar results.
Note that the Schmidt reservoir is the critical dimension and not the rollerball cartridge. Drilling the closed-end hole 1/16" longer to accomodate for variations in the fountain pen reservoir and trimming will make it too deep to provide any spring compression to hold the the tip of the rollerball cartridge against the tip of the pen. That leaves the choices as being to make the pen either rollerball or fountain specific, use 2 springs, or drop a wad of paper in the bottom of the hole to make up the difference. My choice has been to make either a fountain pen with the better Schmidt reservoir or a rollerball pen. I have chosen not to make a pen that can be convertible to either because doing so would require using the shorter stock reservoir, and I don't think that is a good choice for a $100 pen.
Drilling for the rollerball is more critical because you want 3/32" spring compression to hold the tip securely for writing, and any variation of more than 1/64" either way will have you drilling the hole deeper after the pen has been assembled or using that little wad of paper that I mentioned earlier. A couple drops of CA glue down the barrel will make the wad of paper a permanent part of the pen.
A couple things that most people making a closed-end pen miss is that the tube doesn't have to be the same length as the stock kit, and that the diameter past the end of the tube no longer has to be the same as required for the tube. The brass tube only has to be long enough to accept the fitting at the end of the pen. I have been cutting the tube length about 2X its diameter and have not had one come loose when held in with CA glue. The hole past the end of the tube only has to provide clearance for the internals, and there is nothing wrong with having a couple steps in the hole.
The weight of the extra wood partially compensates for the missing metal parts at the other end of the pen, and improves the balance of the pen in the hand of the user. This is really noticeable when the pen is made from a heavier or stabilized wood. The extra wood also allows more freedom to do different things with the end of the pen.
And, don't forget that the length from under the clip to the end of the pen still has to fit in a shirt pocket.
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