.. from what i have seen most people let the seedlings grow for about 1 year before pruning for the 1st time...
Stella, this is not good information nor it's right. To make a bonsai you don't prune a seedling on its first year.
....cut 3 different lengths and see how it goes. depending on the new shoots sprouting you will see which one might end up closest to the pic.
This is also not so good advice...
After the maple trunk thickens (we are talking 5 to 10 years here), then you cut it down (usually to the first branch, if there is one).
OK, you can just "clip and grow" during some years, but it takes longer and you will have to know exactly what you are doing and why.
The good part is that growing from a seedling, there is plenty of time to learn bonsai techniques in the meantime.
And then, after that, you develop your ramification.