Understand the idea
A percentage change compares the difference with the starting amount. Using the ending amount as the denominator answers a different question.
Put it into practice
Subtract the starting value from the ending value, divide by the starting value and multiply by 100. A move from 80 to 100 is a 25% increase.
Know the limits
Percentage change from a zero starting value is undefined under this formula.
The wider context
These guides use arithmetic to explain uncertainty, not to forecast earnings. A hypothetical multiplier is an assumption, not a promise. Real rules, fees, eligibility and outcomes differ. Practice credits have no cash value, and a worked example should never be treated as a reason to risk essential money.
Your next step
Use the practical step above only where it fits your situation. Before an account-specific action, check the current instructions from the service or device provider involved. Write down what you observed and what remains uncertain; that makes a follow-up question more useful.