How to find the loction (loc) of a newly calculated value (not an original data point) on the x-axis(time)?

Hey guys,

I am very new to Matlab and currently on a steep learning curve, so my question most probably will be fairly straight forward for most but challenging for me.

I have a data set that from zero (where the graph starts) to about 2500N (the first local peak - indicated by the horizontal arrow). My data set looks nearly exactly the same as the example image below. I can easily calculate the first peak (2500N) and I know the start point (0). I have to calculate 80% and 20% of the max peak (2500N) during the FIRST steep rise in the data, which is also easy. My challenge is to accurately (if it is possible since it will not be an original data point) calculate the position of the newly calculated 80% and 20% points.

I would greatly appreciate any help!!

Thank you!