Calculating Amplitude and cycle number of point along graph

I have found that for less than 1/2 cycle, a third degree polynomial very closely approximates a sine curve. So select a data range that comes close to 1/4 wavelength around what looks like a peak and calculate the curve then take the derivative and solve for zero. That's how you can find peaks and their amplitude.

Without a workbook, I can't offer much more.

Sometimes a thought has to sit in my brain for a while before it makes sense.

Is this the function you want to solve? A*e^Bt*SIN(Ct + D) + E?

If so, provide some sample data and I can show you how to compute the least squares fit to your data.

I remembered that I had seen this problem before. Almost any periodic function can be described by the general formula I posted above.

The method I use is to take a good guess at what the parameters should be and use a tool called solver. I use my guesses to calculate the values. Then I take the differences between my calculated values and the actual values and square them. I sum up the squares of the differences and that is the value that must be minimized by tweaking the parameters. There is a tool in Excel called Solver that does this kind of calculation.