I can't tell if your formula works. You haven't told us anything about your setup, other than that an ADS1115 is somehow involved.
I'm not sure, but I think that you're using "PSI" as an abbreviation for, "pounds per square inch." That makes me think that you're trying to measure some sort of pressure. That suggests that you have some sort of pressure sensor connected to the ADC. If that's right, then that pressure sensor presumably delivers a voltage that's related in some way to the pressure that it senses. Maybe it delivers a millivolt per psi, or maybe it's a volt per psi, or maybe it's some nonlinear function of psi.
Or, maybe "PSI" means something else in this context. Maybe it's some angle that you hope to measure, and you're using the greek letter psi to indicate that angle. If that's what you're doing, then maybe you have some kind of detector that provides a voltage that's some function of the angle. Maybe it's a millivolt per radian, or a volt per degree, or a half-volt per sine of the angle, or something.
I'll ask again, and hope that you'll actually answer this time: why do you think that this:
int psi = (adc0-5366) * 20000/21353;
will tell you something meaningful about the mysterious quantity, "PSI?"
You can tell us these things, too:
- What kind of thing is, "PSI?"
- What detector are you using? Point us to a datasheet if you can.
- Are you using an Arduino?
- What kind of Arduino?
- What have you done so far, other than type in the code, push the upload button, and hope?