From: east coast, usa
I will look at a few other scenarios to see what they have MRPB at. I am still wondering about a preferred setting here.
My opinion is that it depends on the scenario and how it plays compared to expectations. I believe 99 is the default setting, but in my opinion setting it to 3 at the beginning of the design process is the best thing to do. Then it can be adjusted from 3 if during playtesting results aren't what is expected.
For example, if 10 rounds of combat is resulting in the front lines being too fluid and most units ending the turn in the red, then up it from 3 to 5, or 8. Or if things are too static and it is difficult to get an offensive moving, you could lower it to 2 or even 1, although this situation is usually the cause of some other setting(s) and an MRPB setting of 2 or 1 could make things worse.
So, there is no preferred setting unless you want to consider 3 as the base. The reason there is no preferred setting for this and other settings [such as the Attrition Divider, or Force Proficiency] is that there are too many other factors involved in each scenario's design. Playtesting irons them out.