I think the benefit of ordering a custom piece is that you can have whatever you want and a lot of wearers don't conform to the specs available in a stock unit.
The best rule of thumb when deciding specs is to try to emulate what your hair would have naturally looked like if it hadn't fallen out.
If your hair is naturally lighter on top than at the sides & back (which is quite common) then thats what you should specify.
It doesn't have to be an obvious difference, it can be gradually coloured.
Likewise if you have more grey at the sides & back, then order a higher percentage in those areas, than on top.
The more specific you can be, the more likely you'll get a good match.
Don't forget to send in hair samples from both the darker & lighter areas and use the template to mark out the areas you want specific things in.

I hope this helps.