Hi Clive, Re: Calculations............
Newport uses the larger value of the minimum order value or the minimum shares for the trade to calculate the next buy and sell prices. In your example Newport is looking at the $500 minimum and ignoring the 1 share minimum.
Note that in the AIM Calculator it predicts a Next Buy Price based on just $90 order value where Newport is calculating the next buy price based upon $500.
Same on the Sell side. The AIM Calculator is predicting the Next Sell Price based on a minimum trade size of just $112 of value where Newport takes into account the larger minimum order size of $500.
So the real clue to the difference lies in the minimum order value. Newport calculates everything 'by the book' but looks a which minimum will generate the larger dollar value order. It works on an "at least" basis, too. So, if 5 shares will generate "at least" a $500 order then it will suggest a price that will give us a round share trade on the sell side. On the buy side, 5 shares would undershoot the $500 minimum trade. 6 shares will satisfy the $500 minimum, however.
So, Newport would suggest selling just 5 shares, but would suggest buying 6 shares to satisfy the $500 minimum order size.
Hope this helps.
In the AIM Calculator's case, it would sell $112 worth of stock vs buying just $90 worth of stock. The orders are equal in shares, but not in value.
The reason Newport was designed the way it was had to do with history. Years ago one was penalized, to a degree, for trading "odd lots" (not 100x share lots). Also commissions used to be based upon a minimum plus an amount per share traded. Back in the '80s one wouldn't want to put in an order for 5 or 6 shares of stock because that order would get shuffled to the bottom of the pile. Further with commissions at $100 per trade minimum, one wouldn't consider trading just $90 or $112 worth of stock - not even $500 worth of stock would make any sense.
Neither the AIM Calculator or Newport are wrong.