I agree with Fred -- there is no magic formula. The best Chateau in Bordeaux change their mix every year depending on what that year's fruit is.
His advice to blend the Merlot and Cabernet Sauvignon first is great! I would start with 3 blends: 75% CS/25% M; 50% CS/50% M, and 25% CS/ 75% M. Blind taste test the 3 and pick the favorite.
DO NOT over think this. You will drive yourself insane. And get blasted. Blasted is ok (if you are not driving), going insane is not.
Then take the winning blend and add in small amounts of Cabernet Franc (5/10/15%), again blind tasting.
Exclude or not exclude the Petite Sirah? How much of a purist do you need to be? Is it more important to be a purist, or to blend a better wine? Not that adding Petite Sirah will make a better wine, but it might.
My 2020's are a Meritage blend (66.7% Merlot, 33.3% CS, CF, Malbec, Petit Verdot), and Meritage Plus (40% Merlot, 40% Zinfandel, 20% previous blend). Honestly, the Meritage Plus is the better wine, even if the difference is minimal.
Man, do NOT torture yourself. The enemy of "good" is not "bad", the true enemy is "better". Chasing "better" is a lose/lose/lose situation. If you like your result, you have won!