How to find crystal completion percentage and growth time over time?

Question

Mathematics

Hello!
Help please solve the problem.
There is a website where you can select and grow crystals.
For example, they chose a crystal. We pressed the "grow" button
A value was written to the database - the time the growth began.
Crystals have a growth time (10 days for example).
To make the stone grow faster, you can buy growth percentages.
If we buy 1 percent growth, the start time is moved back a certain time.
How can I find this time and how to find the percentage of readiness that will be after updating the start time.
Perhaps there is another approach to find how much, in percentage terms, the crystal has grown after each page refresh and how much time is left for the crystal to grow.

Answer 1

{start_time} - ( (10*24*60) * 0.01) = new start time
(10*24*60) / ({now} - {new start time}) * 100 = current percentage.