The formula is:
delta-V = g×ISP×Ln(1+fuel_mass/dry_mass)
If you prefer, there are several calculators that allow to calculate easily all that sort of stuff.
For example, if you take that single stage:
View attachment 12948
Its delta-V at full capacity is 2835 m/s. For such a light ship it's not really useful to try to save a few tens of m/s.
Especially that if your slingshot maneuver isn't really precise, you could even spend more delta-V than for a direct transfer
That's why I don't recommend using the Moon for this, this really complicates things for practically no benefit.