Let's say that you are placing a bet for someone to score over 20.5 points at -110 / 52.4% implied.
However, when you go to place the bet, the only line you can get is over 21.5 at +100 / 50.4% implied.
You want to calculate how similar o20.5 / -110 is compared to o21.5 / 100.
Obviously you could go crazy in depth with this, maybe use like a probability distribution to model out how many points you think the player will score, etc. The issue with the proportion is that is makes the assumption that every point is 'equal' in value.
But let's say you did do the proportion. So you do 20.552.4/21.5 = 49.9627907. So the lines seem to be very similar, which doesn't really make sense intuitively, since 1 point out of 21 or so is about 4.5% (1/21100), whereas the pricing for the lines (which are 1 point apart) is just 2.4% different.