OK, I think I understand your questions a bit better now.
As for an overall gear ratio of 40:1 with standard spur gears ... yes, you do have the right idea. A 400 tooth and a 10 tooth do indeed produce a 40:1 ratio. Other combinations will do so. Just look for the "lowest common denominator." It's also possible to go in multiple stages - a 10 tooth driving a 50 tooth (5:1), with the 50 tooth on the same shaft as another 10 tooth, which in turn drives an 80 tooth (8:1). 5*8=40.
The "gotcha" in such an arrangement is backlash - each reduction stage makes its own contribution. Adding more gears can also affect overall accuracy, as each gear contributes.
As for your original test, which produced a 193:1 ratio, you might consider reducing the diameter of the worm gear by a bit more than half, and work toward a 90:1 ratio, This is also a "standard ratio" for dividing heads (4º rotation per turn of control handle). You might have to try several times to get it exact, but it should be do-able. Then find a set of dividing plates meant for a 90:1. I myself bought a Grizzly 90:1 rotab (H2677) a year or two before I found a set of dividing plates to fit it. The part number for Grizzly's plates is G9295, $80 plus whatever shipping they charge.
The dividing plate set includes setup instructions for dividing a circle into a goodly number of even patterns - all spacings from 1/2 to 1/66 of a circle, most from 1/68 thru 1/120, about half of all spacings from 1/120 to 1/180, and some others. I'm sure that commercially available dividing plate sets for 40:1 rotabs will have similar instructions.
In answer to your last question - YES, it can matter a lot. Calculating the setups from an arbitrary ratio (such as your 193:1) and an arbitrary set of dividing plates is possible, but messy. And unless the ratio is easily divided into 360º, very few exact solutions can be found. PM me if you want details of such calculations.