Hi IEAngelsfan,
This would depend on the specifics of your mattress (especially the type, thickness, and firmness of the comfort layers on top of it) and on the specifics of the topper (the type, blend, and firmness of the latex topper). For some people, body types, and preferences, 2" would be a better match on any specific mattress in terms of PPP and for others a 3" topper may work better on the same mattress. While there isn’t a formula that can predict which type, thickness, or firmness topper would be the best “match” for you based on specs (either yours, the topper, or the mattress) or “theory at a distance” with any certainty … post #2 here and the topper guidelines that it links to can help you use your actual sleeping experience as a guideline and reference point to choose the type, thickness, and firmness for a topper that has the best chance of success.
There isn’t an “optimal” blend because it would depend on your preferences, budget, and on the type of latex you are considering (either Dunlop or Talalay). Talalay only comes in two versions which are either a blend that is “about” 70% synthetic and 30% natural blend (some manufacturers list it as 60/40 although it’s closer to 70/30) or 100% natural. Molded Dunlop is most commonly found in 100% natural (although you can sometimes find it in natural/synthetic blends that can range down to 15% natural and 85% synthetic) and with continuous pour Dunlop you find will usually be either 100% natural, 20% natural and 80% synthetic, or 100% synthetic. Each of these can have a different “feel” or properties and cost but they are all good quality and durable materials. There is more about the different types and blends of latex in post #6 here that may also help you choose between them.
Phoenix