I rarely get involved in discussions on here, it seems pointless trying to debate with with people that I don't know who, in some cases, are quite clearly never going to change their opinion. However, as you have said that you are open to accurate calculations I will make an exception.
There is a caveat here that I have had to make certain assumptions (some of which we can be fairly sure won't be correct) so the calculations can't be totally accurate as exact numbers are not know. These assumption are:
When the Old Firm they sell out the South Stand (1654 seats according to Google) with full price adult tickets.
Every extra season ticket sold is an adult ticket, purchased for the South Stand at the early bird price (with no increase to allow for an extra three games being included)
We finish in the bottom six (sorry Tony) and Rangers and Celtic both finish in the top six, so there are three affected games.
On this basis the extra revenue brought in by handing the Old Firm both stands behind the goals is £133974 (1654 x £27 x 3)
The number of extra season tickets required to match this amount of money is 558.25 (£133974 / £240).
Given the feel good factor surrounding the club at the end of the season I wouldn't consider selling an extra 560 season tickets an unrealistic target (not saying it would definitely happen - simply it might). Of course, if we were to consider other factors that number would reduce. For example, if the south stand season tickets were the same price as the rest of the stadium (as they would now include the same number of matches) the number of extra season tickets required would be would be 442.16 (133974 / 303).
Admittedly these calculations do not allow for the fact that some of the people buying the extra season tickets may attend some matches anyway. An alternative way of looking at it could be saying we need to increase our average home support by 320 adults. (£133974 / £22 / 19 games). I don't know what our average home support is but I'd guess this would be approximately 10%. This is another target that I don't think is unrealistic, although obviously we wouldn't know whether it was achieved before the season started and these decisions need to be made.
Finally none of the figures above take into account any savings that the club may make by having less opposition in the stadium or indeed any extra income that they may get by having extra St. Mirren fans in the stadium every match as opposed to extra Old Firm fans for 3 matches. (Approximately 400 x 19 (7600) compared to 1654 x 3 (4962)).