I fixed that dark bit at the southern gate of The Fort.
I made a spreadsheet. I'll explain below...
"Battles" = how many you'd have to make from any one tert to capture all the others. It doesn't matter which tert you start on, it's always the same.
"Neighbors" = the number of potentially enemy terts (other than in your target block) border all the streets bordering your target block. This is important because trying to capture a bonus block that's fairly isolated would be easier than capturing one that can be attacked from all over.
Column G takes the result of result in column E and multiplies it by the square root of neighbors. Neighbors gets that square root because it's less important than column E.
Column H: because column G yields a large number, I raise it to the .36 to reduce it to the penultimate basis for the bonus. That then gets rounded in column I, the actual bonus to be applied.
There are three bases to evaluate this method of bonus calculation:
1) Have I included all the important factors?
2) Does my math make sense?
3) Does the result look reasonable?
HitRed suggests considering the number of armies one would have to beat back. I don't think it matters much. We assume enemy terts have three armies. Roads have two but the one you must leave behind will get eaten by the killer neutral. So it pretty much works out even. So I THINK I've included consideration of all important factors.
We could argue about my math -- what factors to apply or whatever -- for ages. The real test is #3; do my final results seem appropriate? The range from 2 to 8 isn't bad, and every number in between gets used at least once, which is good. So I'd say that this calculation can serve as a good working model until playtest begins. Then we can see what
really influences the ease or difficulty of taking bonus blocks and adjust accordingly.