Does anyone have any tips to help me improve my returns ? Can anyone check my math?

Here are my stats:
https://imgur.com/a/Olkf7

I've been mining Monacoin continuously for 18h using 4x 1080Ti @ 75%TDP, no overclock (257 MHs)

This should net me around 3 MONA coins for 24h according to mining calculator https://whattomine.com/coins/148-mona-lyra2rev2?utf8=%E2%9C%93&hr=257000.0&p=0.0&fee=2&cost=0.1&hcost=0.0&commit=Calculate

So far, I've received 1.5 MONA over 18h, so this comes up to 2 MONA /24h, about 50% less than estimated.

I understand that the calculator may be flawed, and that there is luck involved, but the pool stats seem to say the luck has been good during last 24h.

What really worries me, is the hashrate over the last 18h, as seen by the blue graph, never got close to the 257MHs reported by my miner (peaks at 181 MHs).

I don't have any reject shares and ccminer seams to have accepted 4964/4965 shares over the last 18h.

What can I do to improve my returns ?

Is -50% return compared to estimates normal ?
How can I improve my pool hashrate?