That's a good tip Luke - particularly when modelling (rather than just filling) a large area like you've been doing today, it's all too easy to overestimate the amount of green stuff to mix, and you might as well put the extra to good use. If you don't have enough for a 28mm scale sandbag (if that's all you have left then I'd say you're doing pretty well personally!) then you could always try modelling something smaller like a water-flask, or perhaps sculpt a small rock for basing.
Figures painted in 2016: 4 
"What this game needs is a panda with a chaingun."
"What this game needs is a panda with a chaingun."