Shutter speeds

[stamper]

In a book about HDRI that I bought recently the author recommends that to expose the scene properly you use manual and set an aperture. Spot meter for the sky and the darkest area and note the the shutter speeds. You then pick the shutter speed in between. This I understand. However if I get for instance a shutter speed of 1/2000 and 1/30 what is the one in between? I am guessing 1/250 but my ageing brain isn't reliable. What is the method for doing this reliably?

[pegelli]

The way I would do it is think of it in terms of EV "stops" and then 1/250 is 3 stops away from both 1/2000 as well as 1/30 so smack in the middle.

If you can't do it by heart (I admit, I quickly wrote down the sequence before responding) put the shutter speed sequence with 1 stop apart on a piece of paper, plastify it and put it in your bag to have with you for reference.