If you're not using alpha framework and looking for a way to order quantity based on a percentage of remaining margin without using self.SetHoldings try following code: 

self.stockSymbol = self.AddEquity("SPY", Resolution.Daily).Symbol

quantity = (self.Portfolio.MarginRemaining / self.Securities[self.stockSymbol].Price) * 0.2
self.MarketOrder(self.stockSymbol, quantity)