[Fyron]

The concept is pretty simple. Copy your current design. Add up the 3 resource costs. Divide this total by 1.5. This is the absolute minimum cost you must design the previous step to cost. If your current step costs 10000 resources total, the previous step must cost at least 10000 / 1.5 = 6667 resources. ALWAYS round up. You need to make this step cost as close to 6667 as possible, without going under. Try to use as many of the same components as possible, so as to mimize the number of components that must be added and removed, thus reducing total overall costs. Retroseries design is somewhat of an artform... Once you have this done, you can now build this new design and retrofit it to the target design. Repeat this process to add as many steps as you would like.

Good filler components have a high (total resource cost) per (size) ratio. Master Computers help tremendously, as they are small and quite expensive. Plague Bombs make great filler components, especially for the initial build design. They are fairly expensive and cost only organics, so you can get the resource costs very balanced to minimize the build time of the first step ship.