My water test results are as follows:

fc 4
cc0
tc4
ph 7.5
ta 80
ch 340
cya 50
salt 3800
temp 84

I have never added borates. When I enter this in pool calc it says my CSI is -.26, what does this mean? The chart says -.6 or less is bad for plaster, and .6 is bad for all pools. Should I do something?

I am assuming the SI you are refering to is actually CSI? CSI stands for Calcite Salturation Index which is a measure of how much risk you are at for scaling or pitting (mostly for plaster pools). If you keep your ph a little higher, that will get you away from the -.6 area, or if you raise TA a little.

Before that though, what is your CYA level?

Even with a cyanuric acid of 100 ppm, I am getting a CSI of -0.37 using the numbers that you posted.

I like to keep plaster pools closer to a CSI of 0.0, although that can cause the salt cell to scale. I would say to keep it as close to 0.0 as possible without causing excessive scale in the cell. Perhaps allow your pH to go up to 7.6 or 7.7. Adding borates to 50 ppm will help reduce scaling in the cell and will allow the CSI to be kept a little bit higher. Maintaining a cyanuric acid level of 60 to 70 ppm will also help reduce cell scaling.

Edited my first post to include cya level. I have another pound of cya that I've contemplated adding which should get me to approx 60 on the cya level.
For some reason I thought a lower ph was better so I've been keeping it in the 7.2 / 7.5 range.
I'd like to to the borate thing but I've got a lab that drinks while he swims, alot, my guess is he drinks approx 1/2 gal when he's swimming for several hours, so I did'nt want to posion him.

If you're not getting a lot of scale in the cell, then you could allow the pH to go to 7.6 or 7.7, but watch for cell scaling and back off a little bit on the CSI if the cell scales up.

Raising the cyanuric acid level to 60 ppm will help. Borates are not necessary, so if you're concerned, then it's better to leave them out.

Re entered #'s in pool calc and came up with -.26 csi, not sure what I did the first time. I can see that lowering th ph really lowers the csi so I'll try to keep my ph 7.5 / 7.7 now. I'll add the other lb. of cya also. Thanks for the help.