For under 100 trades he says

sqrt(#Trades) * mean / sd

For over 100 trades he says

sqrt(100) * mean / sd
which is
10 * mean /sd

With regards to SQN ask yourself two questions
1) With regards to SQRT(#Trades). Why cap the number of trades at 100. Is a system that does 120 trades no better than 100? Why is a system that does 100 trades only 15% better than one that does 75, but one that does 75 is 22.5% better than one that does 50?
2) With regards to mean /sd, ie the Sharpe Ratio. Modern finance has shown that downside volatility is more of a concern than upside volatility. It’s very easy to illustrate 2 systems where A has a higher Sharpe than B, but everybody would agree B is the better system.
I believe Van Tharpe has since moved on from it.