how do i adjust the resistance value by parallel rule?

 

Which Resistor value?

The equivalant value of 2 resistors R1 and R2 connected in parallel is

Req = (R1 * R2)/(R1 + R2)

 

1/R1 + 1/R2 + 1/R3 + 1/R4...=1/Rfinal

 

I wish to adjust the value for R1 in order to find a suitable current flow when the battery reach 1.3V and then the LED will bright up.

 

I wish to adjust the value for R1 in order to find a suitable current flow when the battery reach 1.3V and then the LED will bright up.

I suspect you're going to find that using an LED that way will leave a little to be desired. The eye doesn't respond well to subtle changes in luminous intensity.

It would be better to use a comparator and have the LED be either on or off when your desired voltage is reached.

What is your intent with R1? Putting it in the voltage setting path adds some variability in the regulator output; so does your current limit resistor...

BTW, I never liked that type of current limiting. I'd put the current sense on the input of the regulator and use a comparator to "shutdown" the regulator. If I was being really particular, I'd make a -1.25V reference so I could really shut it down.