For the diffuser remember that the body type and the interaction of other modded features will matter. You have a hatch. Babydiesel, I think, might have a notchback. Depending on how precise you want to be, and whether you want to be sure of benefits and not losses, you might consult the below and experiment:
![](http://ecomodder.com/forum/member-california98civic-albums-black++green-picture5282-effect-rear-diffuser-design-angles-drag-according-body.jpg)