sounds dangerous. I'm no expert but I would think the best thing to do would be to remove the last line of tiles. Then draw your plumb line you are looking for and reinstall new tiles that are cut properly on a tile saw.

I think it would be difficult to cut a perfectly straight line with a circular saw through tile anyway, regardless of the balde you are using.

Someone is bound to have a better idea of how to clean that up and I would post pictures if you can.

 

I would not recommend your approach but you can do it with a nice new blade and a corded saw and probably not a cordless one. You will need a friend to keep water on the blade to keep it cooled. You had better be sure you are not in danger of becoming the electrical ground using a circular with water in a bathroom though should something go wrong. I would definitely get a GFCI to plug on the end of the saw or double check the outlet equipped with one you plug it into in the bathroom. Not worth getting electrocuted for this.

I agree that unless you can figure out how to affix some straight edge for the saw to follow, you will probably not be happy with the end result in this. If you did not compensate for the out of plumb situation as you went along so it would not show so much, I am not sure how you fix it at the very end of things.

 

I agree with the note of caution and risk of failure spelled out in posts 2&3.

If you plan to get more tools anyway, here is a solution that needs about $50 in tools.

1) 4 ½” Angle Grinder http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=91223

2) Saw Guide http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=45921

3)Diamond Blade http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41743

You absolutely need some kind of a guide rail to cut the tile. Doing this cut freehand is guaranteed to fail. Use a couple of 2x4's. One section as a guide rail offset plumb to the vertical cut line and the other used as a long wedge against the opposite wall to 'securely' hold the guide rail in place. Use screws or nails to temporarily hold the 2x4's together. Use a short piece of 2x4 against the opposite wall to prevent damage to that wall.

To finish the cut near the bottom of the wall you may have to use a small Dremel or RotoZip rotary cutter.

 

Unless there is some reason you cannot simply remove the tiles that need to be cut, I would mark the tiles, take them down, have somebody with a tile saw cut them and then re-install them. Even if you have to purchase new tiles to replace the ones being removed, I think you would have less cost than if you bought the tools to do this daunting task.


Trying to remember who had it but I think it was Lowes that has a tile saw in their store and will do cuts for you. Don't know what it cost but bound to be cheaper than buying the stuff, especially if you have to put it on a credit card.

I'm kind of lost though because if you cut the exposed end of the tile, what are you covering it with? You do not want an unfinished cut edge of a tile exposed like that.

 

Palibob is right on. I would use the tools he suggested. I just got done cutting some floor tile in between a door jamb to make room for a hardwood floor transition "T". It'll be really dusty, I would cover up anything you don't want to get dusty. I used my circler saw first than I could not get into the corners, than I had to use my Makita grinder to get the edges of my tile. I had to slice into the door jamb to cut the tile all the way. I will caulk the line and no will know, except you fellows.