OP, your logic is sound. Let’s break it down. If you drew 1ml of Test C dosed at 250mg/ml and then drew 1ml of Deca dosed at 200mg/ml the dosage would be exactly what you’d expect them to be.
If you did the same with 10ml of each the proportions would remain the same. Most people choose to buy or make a blend because you could mix them both at a higher concentration in many cases.
Mixing two pre made steroids together is fine. If you’re not doing it in a syringe to immediately inject I’d definitely recommend stirring vigorously with heat and you should always filter everything again and store it in a new sterile container.
Personally I wouldn’t create my blends this way but it is possible, just far from ideal.