I've been trying to calculate based on this equation:
Displacement + [(multipiler 1 * displacement) - displacement] + [(multipiler 2 * displacement) - displacement] and so on. Basically, adding together each multiplier instead of multiplying what you already multiplied. Right?
So a Miata is 1.8 * 1.25 for dohc/4 valve = 2.25 L. Adding a turbo to that will be 2.25 + (1.75 * 1.8 - 1.

= 3.60. Or am I not following the rules?