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Figured I'd chime in here and on some other threads I've been to recently, hopefully if anyone is searching for what I was, they will end up here.  I was having issues taking my vector designs from Affinity Designer in SVG into a program called Sheetcam for operating my cnc plasma.  I could not get the scale to remain where I wanted it.  As much as Inkscape got me started in vector design, I didn't want to have to run my designs through just to make a DXF (which still didn't import perfectly anyway) so I tried using pixels, mm, changing the DPI, but it wasn't working.  In the end, I just did some math to see what the factor was that Sheetcam was changing my designs by.  Obviously I had to make sure the units were the same in each program on export and import as well. My 10 inch square imported at roughly 10.6667.  So I just divided what it should be, by what it was. and came up with .9375.  Sheet cam gives you an option at every import to select the scale to import in.  So Instead of picking 1:1 I make it 1:.9375 and it's perfect.  
I'd like to understand why this works the way it does, whether DPI affects this or not, but I'm content to be able to use my drawings in inches and with the scaling trick.
 
 

I am trying to export SVG files from Affinity Designer and bring them into a range of programs for generating tabs / notches and then outputting for laser cutting. 
 
In Adobe Illustrator it is very simple - you set the document units to mm, design something with specific dimensions, and export your SVG. What comes out will be interpreted by other software, and by the laser cutter at the exact dimensions. 
 
With Affinity Designer, I've spent hours troubleshooting and digging through the forums and no matter what I do dimensions seem to vary depending on the DPI and all sorts of other options, and even slices of objects with the same height will export at different dimensions. It makes it practically impossible to predict what size your object will be. (as an aside, I thought DPI was completely irrelevant to vector - why is it even an option in a vector format?!)
 
So is reliable scale something Affinity Designer just can't do? How are people delivering documents for print if they don't know how the printer will interpret the dimensions of their final files? 
 
I dumped Creative Cloud, bought the Affinity hoping to replace AI and now am faced with going back to Creative Cloud and getting stung with higher fees as a result of leaving. For my line of work this seems like really simple stuff... 
 
 

On the bottom of the stroke panel you have pressure settings.
alt+click to move a single node.