Variable Naming And Looping

I have a sheet which is using a single input value to calculate an output. On the other side of the sheet, I am using goalseek to find what input value would give me a specific output value - this is pre-set.

Starting in row 12, I am using the macro to paste the goalseek solution into a new column at the end and then resetting the original input value. The macro then moves onto row 13, repeats and loops and so on.

My questions are:

1) how can I get the macro to count the number of rows with data in it instead of fixing it at 217 (as seen in "For x= 12 To 217")?

2) how can I get the goalseek goal to reference a user defined cell instead of being fixed at 37 (as seen in "Cells(x, 13).GoalSeek Goal:=37, ChangingCell:=Cells(x, 5)")?

3) after it fills col15 with the goalseek solution to 37 for all the rows, how can I then start a new loop to tell it to start filling col16 with the goalseek solution to say 50, and then col17 with the goalseek solution to say 55? ..