Mike: Indeed, Carl is correct that the altpos grid unit is a direct result of the combination of the screen resolution and font size (aka "DPI") options in the Desktop Settings, although we also apply the dialog font scale factor from the Dialog Sizing dialog.
You can query the screen dimensions and the altpos unit using XCALL AUI, AUI_WINDOW,... as illustrated in the AUIWIN sample program in [908,28]. Here's an excerpt showing the call that retrieves both of those metrics:
! First, lets get the screen res...(CID=-1)
xcall AUI,AUI_WINDOW,-1,HALTGRID,VALTGRID,HALTRES,VALTRES, &
HDLGOVERHEAD,VDLGOVERHEAD,S'TOPSTS,S'BOTSTS,-1,HRES,VRES
? "Ignoring task bar..."
? " Current Screen Res: ";HRES;"x";VRES;" (pixels)"
? " ";HALTRES;"x";VALTRES;" (dialog altpos units)"
? " Dialog altpos unit: ";HALTGRID;"x";VALTGRID;" (pixels)"
As an example, I typically end up with a grid unit of 27 pixels high by 13 wide on my 1400x1050 monitor with normal sized fonts. By dividing the grid height into the screen height, I can figure that I'm going to be able to comfortably fit about 32 logical rows of information (at 100% font scaling) in the largest possible dialog (allowing space for the task bar, title bars, margins, etc.) Similarly, I can figure on about 107 logical columns. Note that the relationship between a logical column and a character is a bit fuzzy with proportional-spaced fonts, but the grid unit width is half way between the "average" and "maximum" character widths, and thus is usually pretty generous, unless you use all capital letters.
What isn't well understood, is whether you can get more effective space without sacrificing readability by increasing the resolution and increasing the font DPI at the same time. I suspect that you might be able to gain a little advantage there, since for a given physical character size, more pixels (higher resolution) will probably increase readability. But unfortunately, you have to reboot to change the font DPI, so it is a major pain to experiment with.
Frank: Regarding why Carl's scaling issues may have been more complicated than yours, I'm not sure there is any one answer, or whether complexity is really the issue. Both applications are visually complex and tightly packed and unique in their own way. I'm guessing that by luck or skill or good karma, you were able to get everything to fit at 800x600 without going below 90% font scaling, whereas, for whatever reason, Carl needed to go closer to 80%, at which point the fonts just become unreadable. (Probably because he wasn't starting from a 24x80 model, like you were, and because he did his initial design on a larger screen.)
In suggesting the switch to altpos for Carl, I was thinking mainly of the following advantages:
1) The altpos system is more self-adjusting, in the sense that changes to the desktop settings (as Mike referred to) are automatically taken into account. With the window-based grid system, the fonts will get bigger or smaller, but your grid will not adjust. (On the other hand, if you're already using every available pixel on the screen, it may be a moot point since neither option -- having the dialog larger than the screen, or having the characters and controls within the dialog overlapping each other -- is accceptable.)
2) The altpos scheme offers an additional scaling option, in which you can independently scale the horizontal and vertical dimensions, and both of them independent of the font size. This is what allowed us to maximize the dialog. (A-Shell just recomputes the horizontal and vertical grid scale factors to spread the controls out accordingly.) In the window-based grid system, you can do something similar, by changing the number of rows and columns that the main window is divided into, or by resizing the main window. But resizing the main window with the mouse is not easy when your dialog already covers much or all of the screen. Note that merely changing the font scale factor changes the horizontal and vertical scale at the same time, so you might end up in situations where you can't use all of the screen real estate because one dimension would over-extend the monitor. Also note that the granularity (or precision) of font scaling is not very good, since we are rounding the grid unit to the nearest integer pixels. But the altpos dialog horizontal and vertical scale factors are more precise because they aren't tied to the font and thus can be applied to overall control size. (Rounding the control or dialog to the nearest pixel is better than rounding the cell grid units to the nearest pixel.)
But, as you say, the advantages are not so overwhelming that it necessarily makes sense to switch, particularly once you have your app working the way you want it.