There's not really an exact formula, since you have to account for grain absorption, dead space in your mash tun, etc.
What I do is drain my first runnings (after vorlauf) into my kettle, which I previously marked with 1/2 gallon increments (you could also mark your mash paddle or stirring spoon like a dipstick). When it's done draining, I know how much I've collected and how much more I need to go. I divide the remainder (the required amount) by 2, since I use two equal sized sparges (better for efficiency). You don't need to worry about additional absorption losses since the grain is already saturated and the deadspace is also as full as it will be later. But heat up a little extra sparge water just in case you miscalculated. I use a smaller kettle that my wife uses for canning to heat my sparge water (I use my kettle for heating the initial strike water).
Also, if you do it this way, you can try first wort hopping - adding your 60-minute hops to the kettle as you drain first runnings and sparge. The hops steep in the 150-ish degree wort and produce about 10% more IBUs, but also contribute a more balanced hop bitterness (search HBT for more info on it). Plus it smells soooo good when you do this.