Calculating dough percent hydration using variably hydrated starters

When doing starter builds, I've taken a liking to building the starter based on percent hydration, yet not always keeping track of just how much flour (or water) is in a given sample. I often use the baker's percentage function on my scale.

I started scratching my head as to how I could use (for example) a 100% hydration starter in an 80% dough without knowing the actual flour and water numbers for the starter. I figured there had to be a formula to easily calculate this knowing the percent hydration. I struggled to find one though.

I excited myself by struggling through an early high-school grade algebra problem and did find an equation, as far as I can tell. I have to assume that it is actually a pretty commonly known equation (looks familiar), and is probably 'solvable' simply by thinking about it with common sense rather than plodding awkwardly on paper through an algebra problem. But despite the bumbling and high-school flashbacks, the equation I came up with is: Flour in starter=total Starter weight/(1+known % Hydration of starter) 

F=S/(1+H)   H expressed as decimal (i.e. 50% as .5)

So I share this in case the equation is not ubiquitous and others may find it useful. But I am also curious...

Is there a simpler (or simply other) way that people adjust their water measurements when using a starter of a different % hydration than the dough?