I'm thinking something like:

200 S-PT @ 1:1 XMR deposited (200 XMR)
200 S-PT @ 1:2 XMR deposited (400 XMR)
200 S-PT @ 1:3 XMR deposited (600 XMR)
200 S-PT @ 1:4 XMR deposited (800 XMR)
200 S-PT @ 1:5 XMR deposited (1000 XMR)

Based on whole number XMR amounts since S=PT does not have fractional amounts, so rounding down to nearest whole number integer.

So in each round minimum deposits for bonus would be:
Rnd 1 - 1 XMR
Rnd 2 - 2 XMR
Rnd 3 - 3 XMR
Rnd 4 - 4 XMR
Rnd 5 - 5 XMR

for example, in first round bonus period:
- if 0.7 XMR deposited - no bonus - floor(0.7)=0
- if 1.7 XMR deposited - 1 S-PT bonus - floor(1.7)=1
- if 1.1 XMR deposited - 1 S-PT bonus - floor(1.1)=1

So bonus would always be based on: floor(deposited XMR amount) * rate

That would mean at least 3K XMR deposited would get some bonus.

Maybe some measures to prevent gaming, like paying bonus after a period of time to prevent immediate withdrawal after the bonus received.

edit: I'm open to suggestions!