How do you calculate AOB - Frustrated!!

[Reece]

Success!! This rusty old fart finally gets it!
One problem I'm having though is speed, each time I have done the Estimate Speed my dial allways shows this ship (Mogami Heavy Cruiser) to be doing 5 to 6 knots.
in the first attempt the speed read 6 knots & the torpedo's just hit the rear end! In the second attempt the speed read 5 knots, I just set it to 9 knots as yours read & this time hit amidships, I tried a third time & the reading was 6 knots again, something is amiss but I don't know what.
This is what I do (I'm sure I got it right):
1) set speed for 1/3rd and hit Z key.
2) go to scope & raise, identify ship, click tick box.
3) lock on, use stadimeter and send to TDC.
4) lower scope, wait for about 1 minute during this time open 4 tubes.
5) raise scope, lock on, use stadimeter, send to TDC, lower scope.
6) go to speed dial & click on "Estimate Speed".
This allways comes to around 6 knots, and I say nuts!
Any idea what I might be doing wrong, can a setting or mod effect this?
Has the Estimate Speed button changed from version 1.0, when I check the SH4 manual page 38 at the top it reads quite different!
Thanks for all your help Fincuan.