You may be confusing percentage increases (bounded by nothing) with percentage decreases (which are often bounded at 100%). Though even decreases aren't actually bounded either -- like when the price of crude oil turned negative at the start of COVID, that resulted in oil experiencing a >100% decrease in cost.

At the end of the day it's all just multiplication, and you can multiply by any number.

If something costs $1 yesterday but costs $2.50 today that is an increase of 150%. How inflation is tracked differs from country to country but typically it's based on looking at the prices for goods the average citizen might buy and tracking the price increases/decreases from year to year.

If a banana goes from costing $10 to costing $26, that's a $16, or 160%, rise in price.

Inflation is "measured" by tracking the price increase of some representative items like food staples or fuel. Prices of course are free to increase much more than 100%.

Some product basket costs $1 at the beginning of the year. At 100% inflation, at the end of the year it will cost $2. At 200%, it will cost $3, and so on.

It’s the increase per year. Prices can go up, say 5x in a year, which would be 500%. Sometimes inflation hits millions of percent.

That would be 400%.

Correct, yes, off by 1 (00%)

Inflation is measured in price increases. So 100% inflation means the price increased by 100%, i.e. prices doubled.