I don't quite understand your problem. Do you mean if X > 3800 you want the screen to show "current = 1"?
I think your logic could be modified a bit as follow:
x = x + 17 ' Progressive increment(??) after every call
IF x > 3800 THEN
j = 0
x = 0 ' you can put many lines between if and endif.
ELSE
j = 240 - X*240/3800
ENDIF
SETLCD 0,1,CHR$(1) ' clear screen
SETLCD 1,1,"Select Current "
SETLCD 2,1,"Current =" +str$(j) +" " ' add spaces to erase old displayed data.
SETLCD 4,1,"Press Enter"
However, in the above case, when X > 3800, j will be set to zero and displayed as zero on the LCD.