Working out a guide number

I've got a Sunpak Softlite 2000A (hotshoe flash), but as it was a

gift and bought second-hand, I've got no information about it.

I've read I need a guide number in order to work out how much to

compensate when bouncing the flash, and using (distance in metres) x

(f no.) at ISO 100, I've come up with an approximate number of 20 -

can anyone tell me if this is correct?

I've also gathered from responses to another post here that Sunpak

are a little optimistic in their published guide numbers - does this

apply here?