I assume you are talking about a thru-neck design. It all depends on what scale length you are using on your build. If it's under 22-23", then a 3' blank is usually sufficient. If it's longer, then it's nice to have an extra few inches for insurance. There are other factors involved, such as the length of the box, whether you are using an angled scarf joint for your headstock, etc.
yea sorry it is a neck through build the box is 8inch by 10 inch & the head stock is 4 inches of the neck, I am a bass player who dabbles with playing guitar so I am just trying to make sure i dont mess the scale up to bad .thanks!