Believe it or not, just use your common sense, it is that easy. Blend the base out and then apply the candy in the same area. If you blend out past the base too far you are going to darken the surounding area.
The more mid coat candy clear you apply the darker it will be right? Well, if you have three coats of candy over your base to match the three coats of candy on the car and then you have six coats of candy just past your base blend then you will have a darker area where those six coats are, it is that simple.
Make yourself a "let down panel" this is IMPARATIVE to match the color. Spray your base out on a spray out card. Then mask off a mask off all but about one forth of the card and spray a coat of the mid coat candy. Then move the masking paper back to expose another fourth of the card leaving the last half of the card still covered. Spray another coat of the candy. This will give you two coats on the first area sprayed and one on the second. Now move the masking paper back to expose another quarter, spray another coat. This will give you three coats on the first area, two on the second and one on the last one. Now pull off all the masking paper of the last quarter and shoot another coat. You will have four on the first area, three on the second, two on the third and one on the last. This is a "let down" card. You will see four totally different colors. You then use this to match the car. See how many coats most closely matches the paint on the car. Check it in GOOD sun light, not in the garage.
You now know how many coats of candy you need to spray over your base to achieve the color. You will also see how adding more candy over your existing paint will darken it. So keep the candy blended out on where the base was blended out and you will maintain the same color.