On a longbow your string length should be about 3 to 4 inches shorter than the length of the bows string groove to string groove measurement. If the string is a little too long, you can shorten it by taking it off the bottom limb and twisting it counter clockwise, if it's too short you twist it clockwise.. Do you make strings?
It's your choice if you want to cut an arrow shelf into a board bow. I would not do that, one it could make the shelf area too thin and your bow could break there. But if you lay the handle area out to be wider, then yes it's possible. A wooden bow just seems out of place to me with a shelf cut into the side of the handle. I make leather arrow rests and attach them to the side of the handle.
I'm sure you will have many more questions and that is fine with us, just enjoy the journey... Welcome to Trad Gang.. Roy