A current quote from the stock exchange includes the following data points:
The highest posted price someone is willing to buy an asset
The number of shares or contracts that people are trying to buy at the bid price
The lowest posted price someone is willing to sell an asset. Also called the "offer price."
There are no rules or metrics for this. Getting a fill at a better price than the natural (say the midpoint or better) is merely a function of a counter party willing to meet you there. If the liquidity is good, it's more likely. If not, less likely.
On active days when the market is moving up or down a lot, traders are active and chances are better that ...