All of Hub's tracks are worth a good look, they are fantastic scenic tracks, and very well done.
In fact, you may want to check out our
Members' Tracks section, if you haven't already done that. There are a wide variety of tracks on show, but it might just give you some ideas.
As for scale, the two major track manufacturers differ on the important matter of lane width, with Scalextric opting for 7.8cm or 3", and Carrera going for 10cm which is closer to to 4". Other manufactrurers go for something in between. All of them also have outside corner borders, because slot cars will wiggle their bums and stick their tails out around the corners when pushed. You'll need an extra 2-4" on the outside of most corners to allow for that. Obviously you are not tied to any of that, since you are building a routed track, but those are your tried and trusted measurements.
It might also be worth looking into the
Routed Track Plans section. There are a wide variety of track plans, which might give you an idea of scale, or perhaps some ideas.
Perhaps
this track might be of interest? It's an 'out and back' track with rally loops, which saves on space, but provides a different challenge at every corner, and allows for elevation changes, which are always good.
I actually think your track design looks quite good, albeit with the caveat that you'll probably need more borders. You've got a decent length straight, followed by a compound curve, and another shorter straight, then a series of corners leading you round the back of track. I would change the radius of one of the two end corners if you want more variation, but your basic idea is good.
If you want a track planner that can handle routed tracks , then
Ultimate Racer is probably your only choice.