Beginner: Axes, Engraving and Scaling

Several general questions - first 2 relating to Marcus Bowman's excellent book "CNC Milling in the workshop"

1)Axes - Marcus has the X axis in his description of a gantry type cnc mill as being perpendicular to the gantry and the Y-axis as being parallel to the gantry. However, my desktop cnc router/mill/engraver machine requested me to setup the machine with the X-axis parallel to the moving gantry and the Y-axis perpendicular. Is there an accepted convention on this matter or do you just choose your machine axes for the proportions of the work and CAD diagram to fit (i.e. if your CAD diagram has an X dimension greater than the Y dimension - then you setup the machine according to Marcus's instruction - or vice-versa - you set up the machine X-Y differently or rotate the CAD diagram).

2) Engraving - Marcus on page 119 in calculating either the width of the cut engraved ine or the depth of the cut engraved line employs "width of tip" as a parameter. To my mind the "width of tip" is totally variable and equal to "width of the cut line" and varies between 0 at the "sharp" end of the engraving cutter to the maximum where the cutting edge becomes the cutter shaft.
The Calculation: "Width of cut line = tip width +2 x depth x TanA (half-angle of cutter)" should just be Width of cut line = 2 x depth x TanA. Has Marcus got this wrong or have I?

Now - nothing to do with Marcus. Is there a relationship between "Motor steps per revolution" and CAD diagrams that can be optimised. In setting up my machine I noticed that changing the "Motor steps per revolution" altered the size of the axes test lines produced - there was a definite one-to-one relationship. Doubling the Motor steps halved the line length - nothing revolutionary there. But 2 things struck me. Motor steps per revolution appears to be a stepper motor variable - is this possible and is there an upper or lower limit to the number of steps?
And if this is a variable - is there an optimum relationship between motor steps and CAD diagram size i.e. you could make a CAD diagram "Big" because its easiest to get the details right and then you use the "Motor Steps per revolution" to "Scale" the diagram down to output size - or if you want to make a "Big" object - just take a normal CAD diagram and use the "Motor Steps per revolution" to make the object bigger. Also I'm guessing that the Motor Steps per revolution is the main arbiter on the precision of the created object - analogous to pixel numbers for screen displays.

Hi Andy - thanks for the detailed answers. Regarding the stepper motor revolutions question: The Mach3 setup that I had to walk through to setup and test my cnc machine requested an input of 400 steps on each of the X,Y,Z motors. When I imported the Mach3.xml file into linuxcnc stepconf - these figures somehow had been translated as 200 steps - why? And the reason I asked about the scaling was that even with the Stepconf "X,Y,Z steps per revolution reset to 400, a test cutting of the linuxcnc example arcspiral.ngc was still half the stated size of the image in the linuxcnc 3D view. So if 400 steps is the preferred number of steps, then are you saying I need to change the Leadscrew pitch value - in my case halve the number of revolutions to achieve the correct size of the cut example. I think I need to go back to the Mach3 setup to see exactly what default values were in that setup for Driver microstepping and leadscrew pitch (or their equivalents in Mach3). In my naive appreciation, as I was only asked to input certain values to get the machine to work, I was unaware of the other variables that may have needed modification. Am I also right in assuming that once I get the correct values into Stepconf for my machine - I don't really have to worry again.

HI JT - very good of you to offer to help me. The attached zip file contains:
Mach3Mill.xml - which is the Mach3 config that I created following the 3020T machine setup instructions.

Mach3Millv2.stepconf - is the linuxcnc conversion from the Mach3 file above (I may have altered the Steps/rev when I found that the scaling was wrong, and I had to add the Estop parport pin link.

ChangeXY.stepconf - is a modification to Mach3Millv2.stepconf that takes into account the replies I have got from this thread - given by andypugh - i.e. the conventional X-Y plane setup and the information regarding no. of stepper motor steps and micro-stepping.

Thanks again for any help you can give.