Affinity Photo Dash Lines Tutorial
Create a line using the pen tool, is the easiest way to demo this.
To aid with this I would suggest view menu and show grid and then go to the grid manager in the same menu and set it to 100 in the basic tab of the manager as I want this to match the 100pt for the stroke width.
Reference / Stroke
Create another line, one as a reference. You should be able to see the stroke settings for the line if you select the move tool but if not, select one of the shape tools. With one of the paths selected, click on the stroke options in the control bar.
Styles for strokes
Go for the 3rd tab, the dash styles. Set the cap to the first option and you get a rounded design, which is fine but to see the spacing and the blocks then it is probably best to go with the second type, butt. Join and align, go with the first option. The butt one is not the only solution but it makes it very clear when you enter 1-1 or 2-1 as the result on the path matches the spacing and block size, the other have the additional curve etc to complicate things.
Note, you cannot use the pressure settings so if you have added a pressure profile then you will find that all the settings for the style will be ignored.
Bottom of the panel 1/1 or 1/2 or 3/1 etc
For the path, default option of 1 and 1 and you will see a block of 100 and then a gap of 100 and then a block of 100 and a gap of 100 and so on. Set it to 1 and 2 then you get a block of 100 and then a gap of 200 and then a block of 100 etc. Go for 2 and 1 then you create a block of 200 with a gap of 100. Go for 1, 1, 1, 0 and then you get a block of 100 and a gap of 100 and then a block of 200 and so on.
You can also enter 0.5 or 0.25 etc or 0.02 etc. If you enter 1-0.2 then the block is going to 100pt (say) and the spacing between them will be 20pt so you will have 100pt / 20pt / 100pt / 20pt etc. If you want to create a wide spacing them enter 20 or 21.2 etc but clearly the decimal makes little difference though it could be useful to fine tune the design. You can also enter 0.2-1 so you have very thin blocks (20pt) and a 100pt gap between them. Or have 0.2-0.2 for a very intense path design.
The phase at the end means you can shift the blocks, so enter 1 and it will shift by 100, enter 0.1 and it will shift by 10 and so on. You can also enter a negative shift. It means you can move the blocks / spacing back and forth to fit with your design especially if you are using this with a shape such as a circle or square as you can then fine tune it to avoid any half blocks appearing.