Provincial Morale Influences

What do the + or - figures given in the morale details table stand for and how are they used to calculate the overall trend?

I have a province showing

Distance to capital 1.6 days: -24
Neighbours: -8
Enemy armies: -1
At war with 3 countries: -15
Enemy neighbours: -15
Buildings:
Fortress: +5

Which comes to 5 - 63, but morale is 67% the trend is shown as rising.

I want to work out exactly what buildings and levels of building to go for to maintain high morale.