It's very short. Someone on these forums estimated it to be about 9", which is probably not far wrong.
Yes, that's pretty much it. Hold up a newspaper, or similar, 9" from your face, and try to read it. Then ascertain what strength glasses are going to help you. I just went into the local £1 shop, and tried a few at that kind of distance, then purchased the best pair, plus 2 others .25 strength either side of the best pair. Then took them home and tried them within the goggles. It worked for me