the other day I bought the Rodenstock DoF and lens tilt calculator and I have to say I am impressed with it!
Although determining the DoF has never been an issue, I had problems working out the correct angle of tilt/swing. With the aid of this tool I did some tests and I got it bang on right every time. Of course the angle had to be slightly adjusted, but when for the first time I got the everything on focus along the plane of sharp focus, even at f/5.6, I was really a happy chap!
Probably, with more practice, I won't need the tool anymore, but I have to say that's worth getting one, especially when more precision is required, e.g. when shooting close ups. So, I definitely recommend buying one, particularly if you're a newbie like me. But I daresay even more advanced users can find it useful.
Also, and this is very important, it only costs £22.50 at Linhof & Studio, unlike the £42 you would have to pay at Calumet.
Valerio