1-2 spread
You would need a good game. You can make a profit flat-betting or with a small spread in good games. In a six-deck, S17, DAS game, a player with a $10K bankroll who can find a game penetrated to 5 decks can make $23.08 for every 100 hands seen using a unit of $64.10, playing to a 5% ROR. If he uses a 1-2 spread, his unit would be $43.29 and he could make $29.87 for every 100 hands seen. Add late surrender to the same game, and a player with a 10K bankroll who wants to play to a 5% ROR could use a unit of $80.65 for a profit of $34.68. A 2-1 spread can be played with a unit of $52.91 and an expectation of $42.33 per 100 hands seen.
Obviously, some adjustments would have to be made to play to whole units, but it is an example of how a Wonging approach can be used. It should be pointed out that your profit per hand played is much higher, but I am talking about hands seen, not necessarily played. One good thing about the higher bet needed for Wonging is that, with a large enough bankroll, you will get comp offers immediately and will get mail offers as well, if you choose to be rated.
My book will have an extensive set of charts that lay out the bet spreads, the betting units needed and the expected value for a variety of shoe games using the Wonging style. It will also have similar information for the use of an exit strategy, which can be played profitably to a 1-4 spread in six deck shoe games.