when I design a plane from scratch, I aim to have the CG be 25-30% back from the leading edge of the wing. IE, if my wing is 6'' wide then I try to have the cg be around 1.75'' from the edge.
typically, this can be achieved by finishing the airframe and attaching the motor/prop and then placing the battery and servos in places to get that desired CG. putting a elevator servo on the tail, or in the fuse with a long pushrod can adjust CG. so if I check CG with just the motor installed and it is tail heavy, ill put the servo in the fuse with a long pushrod. if it seems nose heavy, ill put it on the tail. Tho, most of the time I can make the needed adjustment with the battery's placement. I don't make the battery tray until I know where it needs to sit to get the CG I want.
If you place all electronics and battery first, then the only way to get the CG you want is to add weight (to the nose or tail, depending on what adjustment is needed). There is nothing wrong with that, as long as you don't have to add a ton of weight.
Also, I found some pics via a google search that showed a basic high wing plane design and it gave the ratio's that generally make a good plane. here they are, they will help. I could have avoided a few failed designs had I found the pics earlier.