[Dedas]

"Increase" and "spread" are two different tests:

The "Increase" test is as you described. If that test is failed for (example) a temple no "spread" test will occur.
But when the test is passed there will be another test called "Spread" (if not the province is neutral, then it will increase automatically by 1).

The spread test is as follows: 30% minus (3% multiplied by the current friendly dominion in that province).

If that test fails the dominion will try another province until all is tested or it bumps into an enemy dominion. The test for converting enemy dominion is 50% plus (your maximum dominion strength times 5%) minus (current enemy dominion in the province times 5%).

My point is:
The more dominion you have in a province the harder it will be for it to increase even if you have 1000 temples and 10 in starting dominion. Thas is because the the spread test is not affected by above factors, only your current dominion in that province and that is acting as a deterrent. 30% is the highest spread value you can have. 100% is the highest increase value you can have (I believe), needed to take those spread tests.

Cheers!