I have just purchased a Hoya ND x400 recently to take with me for a trip to zanzibar - so i am in the same boat as the OP - here is one article i found very useful. Hope this helps others:
Brilliant Article - Here is a quote from the Author on Calculating Exposure:
The purists are going to cringe when they read this, but when you are composing and focusing your shot before screwing in the BG, I wouldn’t worry about the exposure. In other words, don’t stress about having to calculate how many stops of overexposure you need to get an evenly-balanced exposure after putting the BG on. I have tried to do this and have never succeeded in getting anything other than an overexposed blob. I have a theory its to do with light intensity. And besides, one of the major advantages of using a BG is that you do not have to adjust ISO to compensate for bright light and instead you have greater control over aperture and speed. I discuss ISO and BGs more below, but my point is this: if you want to use ISO 100 just do it. If you want to use f22 or f2.8 or whatever, just do it. You can because what’s left is speed, and the BG gives you enormous range in that regard to find the right exposure when using the BG for any particular shot.
Another reason for not worrying about the exposure is that f stop compensation calculations assume you won’t also be using a Grad ND, which will darken your image. Try doing the calculations for that!
In the absence of doing those exposure calculations, it is a case of trial and error. 60 seconds and your image is still dark? Well shoot again, this time at 90 seconds. A little too overexposed? Well try again, this time at 75 seconds. Each time you are narrowing down the exposure time. The more you use the BG, the more comfortable you’ll get with gut feel. Bear in mind that if you shoot at, say, 300 seconds, adjusting the next shot to 315 seconds is not going to make a discernable difference. A 15 second differential will, however, make a difference between 60 and 75 seconds. Also bear in mind that the person I consider to be the Monochrome King of the BG (see below) admits that success with long exposures is “trial and error”, just like I said.