"I suppose you inserted your general function as a User defined model into "Insert trendline" dialog. By default, initial values for your constants are $b=1, $c=1, $d=1, very much different from resulting figures."

you are correct, i did do that.

from our raw data I can easily find the $b value

but here is the problem

i need to find the $c value from the best possible graph aka. R^2=1 (or very close)

and i find the best graph by finding the best $d value. with is a little different from every experiment we made

and even with only 2 variables it still come with an error

i can inset random numbers until i get close but it will take to long.

you say "The program runs 100 iterations to find reasonable results", can i force the program to make more?