What is the optimal ratio of Steamworks to Magnetos, regarding blueprint cost?

Magnetos boost productivity by 2%. Steamworks boost this 2%-bonus by 15%, or equivalently 2% * 15% = 0.3% per Magneto. So when I have 7 Magnetos, buying 1 Steamworks gives a larger boost than another Magneto.

However, both cost rare resources, especially blueprints, which are tedious to obtain. For example, at 10 Magnetos and 11 Steamworks, the former cost 9.11 Blueprints and the latter 11.64 Blueprints. Additionally, I'm not sure if there are diminishing returns on either productivity bonus.

So, which ratio of Steamworks to Magnetos should I maintain for maximum productivity boost per blueprint spent?