Hi Grapefruit
How many tabs are you using per line?
If you put a left tab at say 10 picas with no fill characters (underline) and then put a right tab at 20 picas with underline fill (___) you will have a line or rule starting at 10 picas and will be 10 picas long and end at 20 picas.
If you want the rule all the way across the page you can hit option tab and then choose the underline and apply a right tab close to the right margin.
If you use tab and underline to make a rule across the page it is normal for the left side to be indented a point or two, to eliminate most of this indent highlight the line and go to tracking at the bottom of page and hold the option key and click to decrease tracking by -1 or -2 or whatever is necessary, this should move the line to the left and eleminate any space between the rule characters.
Also if you want a rule across the page why not use Rule Below the line and then you can control the indent and the line will be the exact width of your page or whatever indent you choose.
Hope this helps.
IMacQuarker