I have been using a program call Prop Power Calculator to see what a particular engine is capable of . Icalculate the thrust and speed based on what rpm an engine will turn a particular prop (all props are not created equal) Itry to use engines that achieve somewhere around 1 to 1 thrust to weight but most planes will fly just fine with a lot less depending on their wing loading.
Calculating for speed depends on the plane , a trainer flying at 40mph is happy enough but you may not want your P-51 to go that slow. You end up trading thrust for speed in most cases. And vice versa.