I calculated the ideal (one) column width / line length for 9 and 10 pt Helvetica Neue Light.
I based a three column grid on the above calculation… The main text column width equals / spans two grid columns, the other column is for image captions.
Now… even with ample margins the main text column looks really narrow (on 312 x 237 mm paper)
If I increase the column width till visually pleasing, character count per line ends up way above acceptable.
Two narrow columns (each equal to one grid column) for the main text also doesn’t seem to work as there are too much long words / ends up below acceptable character count per line.
What are my options besides suggesting a different paper size?
You seem to approach aesthetic problems with the eye of an engineer rather than an artist. Good graphic design requires a balance of each plus a whole lot of gut instincts and judgment calls that run counter to adherence to rules of thumb and formulas.
You need to change your grid for optical adjustment. You’ll learn quickly that math doesn’t always work in the design world. Many layouts and logos look symmetrical but are mathematically unbalanced. I’d suggest adjusting it so that it works visually while ignoring the measurements. You’ll then be able to measure to see if it’s possible for a new grid… where perhaps 2 colmns are the same size, but the center one isn’t.
I can totally see why it you would say that.