Two simple thoughts.
Don‘t go right to the edge of the baseboard, allow yourself a little bit of leeway in case of mismeasuring or wanting to juggle things slightly. This also reduces the chance of your favourite loco taking a dive.
Your arms have only a fixed length, so make sure you can reach any part of the layout for cleaning or rescuing a derailed vehicle. The same applies to making sure you have access, hidden if need be, to the inside of any lengthy tunnel.
Welcome to the forums
@phaywo! If you're wanting to design a layout yourself, there are some good programs available for download online which can help. There are free versions of both, but you'll need to pay if you want to use more than a certain number of pieces.
Here's a link to AnyRail:
https://www.anyrail.com/en
And here's a link to SCARM, which I used for planning my layout:
https://www.scarm.info/index.php
One bit of advice I've got is to resist the temptation (should there be one) to fill the board with track. I did that with my layout initially but found it a lot more fun when I took some of it up and replaced it with important scenery such as a bakery and model shop.
-Peter
I agree thoroughly with the above two bits of advice that I've put in bold.
Does the 70cm x 170cm need to include some sort of fiddle yard space too, or is there additional space for that even if it's only attached temporarily?
Looking at the space you have, if you're prepared to do an end to end you can squeeze some sort of linear end-to-end shunting yard, depot or station layout fairly easily. If you look at my 'Fort George' layout, you'd have basically from the White farmhouse to the left and as you can see you can get a fair bit in that space.
However, if this plan that you have done fits in the space, I'd be tempted to use it as a blueprint:
I'd be tempted to remove the inside blue circles of track, use the purple points instead to create a 'passing loop' at each side of the layout and then put a platform in the middle of one, along with a signal box and an industrial building. You can then split the layout in half and one side is the scenic side, the other the 'fiddle yard' side. You could even have a small siding coming off the passing loop. This would give you quite a lot of operational interest, allow freight and passenger operations, shunting and through trains all in one layout. It would actually be quite similar to
@Cowley 's Lapford layout, who shows what can be achieved in a similar space.