You know, you have two choices here. There is clearly long edge. One thing to do is simply to go long. But perhaps a better way to trade is, say your natural units is to buy 500 SPY. Well, I would buy 200 SPY, and if it goes to the natural support, buy another 300 at 1272 ish (assuming FV doesn't go premium). If SPX goes higher to FV without full position on, at least you had 200 hundred on. C'est la vie.
There is no right or wrong as far as what part of your position to have on at one time. I think that the longer the timeframe though, as FV requires to trade it, the more cautious you have to be with position sizing trades. It is a campaign.