Spacing out

I have been asked how do you determine the spacing of your trim work , my sister has been battling with just this question too .
Well I have a long wall in my bedroom , so I started there , I found the center point , and the the center of each half , and then the center of the next two sections , is that a clear way to explain it ??? I am not so sure .
Ok I will try again , I measured the entire length of the wall divided that in half and found my center point , then from the wall to the center point I divided that section in two , then so on and so on , until I got the desire visual spacing , then then I did the same on the other longer wall , it turned out to be every foot I centered a vertical board .
I then cut two boards at 63 inched and propped them up on the baseboard , and used that as my helping hand to hold up the top rail while I nailed it while making sure it was all level .
But since the baseboard may be a little lower or higher in areas {because I have carpet in this room for now} my goal was to keep the top rail level .
So you can not just go out and cut 40 pieces all the same size for the vertical pieces , these boards are best marked in place after the baseboard and rails are in place , time consuming , yes , but this is how I did it , and do it always.
I then always use a level to check that the vertical pieces are straight , this will ensure that everything stays square.
Then I fill all the nail holes , and caulk any gaps that I could not avoid { and that is OK in my book} it doesn't have to be perfect to be beautiful.
Next step is to sand everything smooth , vacuum and then get to priming .
Priming and more priming .
And then painting , painting and more painting , which is why I estimated a week to get this room complete , I am still thinking by Tuesday or Wednesday I should re-claim my restful bedroom .
Ah that will be nice .
Keep the questions coming , I am happy to help , that is why I started this blog after all .